Changes imported from Abseil "staging" branch:

  - 06c8c67f5a564d00696e023060f05a5c34e7e164 IWYU | absl/base by Juemin Yang <jueminyang@google.com>
  - 2b1a054a09bda55843b449843b2a125741e936e7 Internal refactoring by Greg Miller <jgm@google.com>
  - f43f7f1f91bee26b5ddcd0c5bbbc47cb977aef77 Make std::hash<absl::optional<T>> to be standard compliant: by Xiaoyi Zhang <zhangxy@google.com>
  - 539bad2ebc22e610e1f292285a30a87945bc663e Update utility.h to Abseil standards by Tom Manshreck <shreck@google.com>
  - d05ec10a5f16a5d6640e0db91ecc7ab3ea971fd4 Add a test for absl::Barrier. by Derek Mauro <dmauro@google.com>
  - d707e27acb3c06f0d74c5f7ad7861e3841a5471f Run leak-checking tool over all outbound code. by Daniel Katz <katzdm@google.com>
  - 55f07f482a50422b8f99f7176374a19d0d473c5f Add alignas(16) to uint128. by Alex Strelnikov <strel@google.com>
  - 94999b7edde82308f736fb939501537ee9edbca6 Update attributes.h to Abseil standards by Tom Manshreck <shreck@google.com>
  - 321bed0061c41b53d0206ad4865528c00dd6d825 Test git merge + piper cl process by Juemin Yang <jueminyang@google.com>
  - 69920e7351a1053a7f4940bbde1768e839ce84bc Adds support for "/etc/localtime" as an embedded time zon... by Greg Miller <jgm@google.com>
  - 6839c06bf232903d3a9cbffa6eb2c960db78e67b Add copyright notices to inlined_vector code. by Greg Falcon <gfalcon@google.com>
  - 4e2714f6266263515cdfd31675c30c6ed6f98e1a Adding Apache 2.0 License by Gennadiy Civil <misterg@google.com>
  - 7402e7594016a4cd0a8b823fe6bc1bad1874bb85 IWYU | absl/utility by Juemin Yang <jueminyang@google.com>
  - 271a3812337eed97c412042738482688a80e19bd IWYU | absl/memory by Juemin Yang <jueminyang@google.com>
  - 32bda13a8098c2b06e25a5cf7bb782d6b79eb006 IWYU | absl/numeric by Juemin Yang <jueminyang@google.com>
  - 62d375cedc133108904bc06e340e303091a565df Remove "no_test"-annotations on span_test_noexceptions. by Daniel Katz <katzdm@google.com>
  - ebcbae9a55a93a7f1bb6862edc2715a6d9877206 Move CI-testing support files out of public-facing reposi... by Daniel Katz <katzdm@google.com>
  - d3f05eff4daa6030bfacb31cca0f9213fb702247 Fixes ToInt64Minutes() and ToInt64Hours() to properly sat... by Greg Miller <jgm@google.com>
  - b8dfae3facb6bb002622f083a10d14448f19e6e0 Fix typo. by Abseil Team <absl-team@google.com>
  - 150f03baa0afa231c2fc01597ea2321da586caba Update README.md by Yilei Yang <yileiyang@google.com>
  - 05276aa837dd081686518fd27bda4bd206ac4443 Adding Apache 2.0 License by Gennadiy Civil <misterg@google.com>
  - 37bf8e223e79ad06a195e28db9499e0c3d140f73 IWYU | absl/container by Juemin Yang <jueminyang@google.com>
  - 49164928f220978a32f88d16a55549bdf871daef IWYU | absl/algorithm by Juemin Yang <jueminyang@google.com>
  - a6804734e129039f9580a4fcd0f66425d0d0ac30 Move throw delegate wrappers to an internal namespace. by Greg Falcon <gfalcon@google.com>
  - ac83e73f67f593e2aff957b2be0b28e59c552a71 Fix error in comment stripping directives. by Greg Falcon <gfalcon@google.com>
  - e018a24185a984e787fb81a75fc35b74ad3a4d3d Update copyright headers all BUILD files by Gennadiy Civil <misterg@google.com>
  - a3be0990bfd76b0dec76bd85cecfa4dcec68b3ea Fix closing namespace comment typo. by Abseil Team <absl-team@google.com>
  - be3e3c4327e4f83949e0f29fd7a190d7eaa8b50b Update TODO by Abseil Team <absl-team@google.com>
  - f56a5d6f72685d92bb9c2905841b950d8177210c Add test-coverage for leak-sanitizer. by Daniel Katz <katzdm@google.com>
  - 7694bf161c7e00fdd08bfadc2aaf8e0fb09335f8 span.h: further touch up wording around the std::initiali... by Abseil Team <absl-team@google.com>
  - 3a12e081c0f8b359973e020d1e91f65356548ebc Update time.h to Abseil standards by Tom Manshreck <shreck@google.com>
  - 48d28f6468129420f4b20d451dca8e08012a7a77 Remove references to google from comments in Abseil. by Greg Falcon <gfalcon@google.com>
  - 773e34402d15fcad6370d5ed2430482d17db910d Rename the ExpectTime macro to match Abseil naming conven... by Greg Falcon <gfalcon@google.com>
  - 774d2ff1fe26c7313b301ff203e83e1aaac86627 Internal change. by Daniel Katz <katzdm@google.com>
  - 2e8a5830e95c8a1b839721bb2f1d4f5c85b9fb60 Fix typo in comment (missing '*' on a pointer). by Abseil Team <absl-team@google.com>
  - 458106feb707cf9609dd243713bde44aa9679e2a Correct capitalization: github -> GitHub by Yilei Yang <yileiyang@google.com>
  - af440725f02c2a83ca5cbaf176e1142f9e9d9b2f Update copyright headers by Gennadiy Civil <misterg@google.com>
  - 05b1118cce4ab87d23c33d48e64a96bcfec08761 Update copyright headers by Gennadiy Civil <misterg@google.com>
  - d5c6669a62d047156bb77055c5da03ee1b3c61b9 Update Abseil README to include descriptions of the inclu... by Tom Manshreck <shreck@google.com>
  - 3cd7e4663dddc840087469a6495f6cf433bfad8d Update copyright headers in //base by Tom Manshreck <shreck@google.com>
  - 7a876da657cd6698c5da2008a582d52eedc85dd1 Update strings overview with robust string library docume... by Tom Manshreck <shreck@google.com>
  - d9e3d0768d6f1c77d30992bdbef7b47ec92994bb Update copyright headers all BUILD files by Abseil Team <absl-team@google.com>
  - 6fe942728bceb0625f7c79b2840c4a6154d076b3 Make InlinedVector, FixedArray, and Span's at() throw on ... by Jon Cohen <cohenjon@google.com>
  - 5b52d5ec6cb9fbb07fc2e2fa020bd3eeb48c4953 Update clock.h to Abseil standards by Tom Manshreck <shreck@google.com>
  - c03c1ca3aee8bb7e40aa0315f6c432d31a72c30c Update //algorithm copyright headers by Tom Manshreck <shreck@google.com>
  - d46f40ddc596aaacb0459351d0e4aa6871289fa2 Temporarily prevent running mutex_test on crosstool17, wh... by Jon Cohen <cohenjon@google.com>
  - 61f11476189df68edfb9908308d677a91f03ff67 Update copyright headers in //container by Tom Manshreck <shreck@google.com>
  - 91832c00948954edf0b3dda12219c9a0202421ac Update copyright headers for //synchronization by Tom Manshreck <shreck@google.com>
  - 4e09100264b4585af6b4508ff35b9c627ac1f1ce Update copyright headers all BUILD files by Gennadiy Civil <misterg@google.com>
  - 13a0e8aebedec0f95b33750cbcd6b5548619b2a5 Update copyright headers in //memory by Tom Manshreck <shreck@google.com>
  - 63e1b9d4fdbcdf097e5276050ad1f76f0053e553 Update copyright headers for //strings (+ one from //nume... by Tom Manshreck <shreck@google.com>
  - 0108e7cfc50777a94c56d00e9c305161364df341 Convert ASSERT and EXPECT to ABSL_RAW_CHECK in helper fun... by Derek Mauro <dmauro@google.com>
  - 0122306fe47a3093248254a1b475c3a1d82abec5 Internal change by Abseil Team <absl-team@google.com>
  - 89c0c2698c98a12cd63172eeb02063b2f67e7c81 #absl Fix comment. by Abseil Team <absl-team@google.com>
  - 6621cc1ff54800e0aadb5e3071dbaa84b2077ceb Publishing contributing guidelines. by Gennadiy Civil <misterg@google.com>
  - e48c5be3c75e794b3e3827d40915b01fe1a1afc5 Avoid PRIdPTR.  Cast to long long and use %lld insted. by Abseil Team <absl-team@google.com>
  - 2640ea4a260d89b94b07a3142660327e47db33fd No algorithmic changes. by Abseil Team <absl-team@google.com>
  - 1bc6c1bad17754f5d84963bf1d0db279402a0a1d Internal change. by Derek Mauro <dmauro@google.com>
  - 6845d24733e8c95bebde825ba78a2abfd9e35bdb span.h: fix up incorrect wording around lvalues. by Abseil Team <absl-team@google.com>
  - d8f5caee721e252e5f9b1080fb996363f498ac28 Add more exaustive Mutex testing. by Derek Mauro <dmauro@google.com>
  - e8b4cb053eb98858eef10cc53280b6ed5d6815a7 Change Span::subspan to not call into a deprecated constr... by Jon Cohen <cohenjon@google.com>
  - 49c36a82b3114926390557670aaaf0ea25b5760c s/std::size_t/size_t/g by Jon Cohen <cohenjon@google.com>
  - e17487c3c4d4a99f2fd8bc3e42176fc3171614d1 Account for the case of timeval::tv_sec being smaller than by Abseil Team <absl-team@google.com>
  - ecbb89d5fb98483e777c03d97ac02d7b7b54985e Alias absl::string_view to std::string_view when C++17 st... by Xiaoyi Zhang <zhangxy@google.com>
  - 6820e5a51459cdbb6a423cbae25a0cc839c85d44 Internal cleanup. by Xiaoyi Zhang <zhangxy@google.com>
  - 6976469b76a6faaf4111a24ddb37f40211ffadae More Span constexpr by Jon Cohen <cohenjon@google.com>
  - 8521c8956eee1125b7759eb272ec4a5a86fcefc5 #absl Fix comment. by Abseil Team <absl-team@google.com>
  - 20eae7a67fde5dd809aa47e5f7de8a493701645e Embed enough zoneinfo data to make time:time_test (under ... by Abseil Team <absl-team@google.com>
  - 841f5d98ceef4a423839ea73ee06c2f47a9b9680 Clean up macros in attributes.h | ATTRIBUTE_INITIAL_EXEC by Juemin Yang <jueminyang@google.com>
  - 83d8b36656e47919b5d0bac82eece897e195697e Update any.h comments to Abseil standards by Tom Manshreck <shreck@google.com>
  - bb3fae11d3459eeae2f63bfd22e65d3193187cc8 Update type_traits.h comments to Abseil standards by Tom Manshreck <shreck@google.com>
  - 992e1b07c0dec64271f8c44f22fd8df3734d0c47 Renamespace CycleClock code. by Greg Falcon <gfalcon@google.com>
  - 08d6fb0594098493ffbc0e737405182638122e7e Eliminate more existing lint warnings by Gennadiy Rozental <rogeeff@google.com>
  - 1cc6fdc71eb777497239f8c3e9168e6c9d40ea53 Update optional.h to Abseil standards by Tom Manshreck <shreck@google.com>
  - baa91747aa55009a9eb31b6072e33db06cfce2d2 Enforce internal namespace symbol reference policy by Gennadiy Rozental <rogeeff@google.com>
  - 939251e39342ce559e5d23fe43799671581f7cf5 Add CycleClock scaling shift to mitigate a possible overf... by Derek Mauro <dmauro@google.com>
  - c6dfdeecea0c7470938bed47c99ea2b2a95889d8 Add constexpr tests for absl::make_optional(). by Xiaoyi Zhang <zhangxy@google.com>
  - 509e949b992db33041d840746fbd05cc01cb206e Alias absl::optional to std::optional when C++17 std::opt... by Xiaoyi Zhang <zhangxy@google.com>
  - a1ae6d96a8826ba75281cac8632a766b5856acaf Remove no_test_* tags from span_test to increase test cov... by Xiaoyi Zhang <zhangxy@google.com>
  - 3c2a43cc09791723c8a324836629644ac44cb9c8 Remove accidental bits of Google-internal code and short ... by Greg Falcon <gfalcon@google.com>
  - 4874d49d496ac0b6ec36f4280a14b2159e7af930 Replaces the macro-generated Duration factory functions e... by Greg Miller <jgm@google.com>
  - 90e62695e03cb4a57e137ca0c3e116b1d802db57 Fix namespacing for a couple files in base/internal, and ... by Greg Falcon <gfalcon@google.com>
  - b0e6e00e34f967924849aaf8c123bba068f093e3 Publishing contributing guidelines. by Gennadiy Civil <misterg@google.com>
  - d74eafbccc3dffa6c25f9b6a2219425a24b5a959 Internal change by Abseil Team <absl-team@google.com>
  - 27477badbbf720265f5b9509b6c0e01913dc0a9f Update escaping.h comments to Abseil standards by Tom Manshreck <shreck@google.com>
  - 67002f55738319c2875197c3b6282de215ec250d adds absl namespace to debugging/stacktrace.h by Behzad Nouri <bnouri@google.com>
  - e608018f7faa384d5b202ac0a4c7a0d5166f4d9c Update string_view.h comments to Abseil standards by Tom Manshreck <shreck@google.com>
  - e884f04d4c648e01ed7dcde2fda80c24e8452047 Exclude strings/ files we are not releasing from OSS univ... by Gennadiy Rozental <rogeeff@google.com>
  - 3f4c4032ed520f2dd10a81d58ef4f399c001c5cc Strip out eventmanager reference from release. by Gennadiy Rozental <rogeeff@google.com>
  - fb0f1c204793c3792bad101dbaa734e7c2a35887 Fix copybara strip comments by Gennadiy Rozental <rogeeff@google.com>
  - 3eaaac942f77c3d41d63d414630403bfd0f6b70c Strip out style guide waivers. by Gennadiy Rozental <rogeeff@google.com>
  - 020e045058173178b51266b99a2a5dc9ed921960 substitute_test portability | MUST_USE_RESULT cast-to-voi... by Juemin Yang <jueminyang@google.com>
  - 86c093bf81d80ff537ed5e8b89225ce75a636220 Internal change. by Derek Mauro <dmauro@google.com>
  - 330375eb952fe78276e75631a28e750d5bfdb198 Prefer absl::FixedTimeZone() over loading "Etc/GMT[-+]<N>". by Abseil Team <absl-team@google.com>
  - 2e07ebee46a8201adc0dfd2c4ddb3df76e524357 Internal change. by Derek Mauro <dmauro@google.com>
  - 1f0c8b78c8ebd66f14cdf39fcba9f4c9986dcdca ::absl -> absl by Gennadiy Rozental <rogeeff@google.com>
  - ad163566d12ea08f1da2c23931eeacfffc564139 Avoid old style loops where possible. by Gennadiy Rozental <rogeeff@google.com>
  - bce2108818fe57b5617ce0090ddd4f753808f0a1 Update comments in str_cat.h in line with recent changes. by Abseil Team <absl-team@google.com>
  - cfd593a80f4897256f2ce1ea0be55dc14e3fcad4 Copybara-out gtl aliases. by Gennadiy Rozental <rogeeff@google.com>
  - 584f1524d717993c1a16093caccd9ed2b1e5409e Fix a warning for Windows/Kokoro time_test.cc. by Daniel Katz <katzdm@google.com>
  (And 562 more changes)

GitOrigin-RevId: 06c8c67f5a564d00696e023060f05a5c34e7e164
Change-Id: I89907a6188fe7de05da400bf49ddfeba242aff8e

2 files changed, 79 insertions, 9 deletions

diff --git a/absl/types/optional.h b/absl/types/optional.h
index 5099d489..3e010bd5 100644
--- a/absl/types/optional.h
+++ b/absl/types/optional.h
@@ -439,6 +439,33 @@ struct is_constructible_convertible_assignable_from_optional
 // for checking whether an expression is convertible to bool.
 bool convertible_to_bool(bool);
 
+// Base class for std::hash<absl::optional<T>>:
+// If std::hash<std::remove_const_t<T>> is enabled, it provides operator() to
+// compute the hash; Otherwise, it is disabled.
+// Reference N4659 23.14.15 [unord.hash].
+template <typename T, typename = size_t>
+struct optional_hash_base {
+  optional_hash_base() = delete;
+  optional_hash_base(const optional_hash_base&) = delete;
+  optional_hash_base(optional_hash_base&&) = delete;
+  optional_hash_base& operator=(const optional_hash_base&) = delete;
+  optional_hash_base& operator=(optional_hash_base&&) = delete;
+};
+
+template <typename T>
+struct optional_hash_base<T, decltype(std::hash<absl::remove_const_t<T> >()(
+                                 std::declval<absl::remove_const_t<T> >()))> {
+  using argument_type = absl::optional<T>;
+  using result_type = size_t;
+  size_t operator()(const absl::optional<T>& opt) const {
+    if (opt) {
+      return std::hash<absl::remove_const_t<T> >()(*opt);
+    } else {
+      return static_cast<size_t>(0x297814aaad196e6dULL);
+    }
+  }
+};
+
 }  // namespace optional_internal
 
 // -----------------------------------------------------------------------------
@@ -1072,15 +1099,8 @@ namespace std {
 
 // std::hash specialization for absl::optional.
 template <typename T>
-struct hash<absl::optional<T>> {
-  size_t operator()(const absl::optional<T>& opt) const {
-    if (opt) {
-      return hash<T>()(*opt);
-    } else {
-      return static_cast<size_t>(0x297814aaad196e6dULL);
-    }
-  }
-};
+struct hash<absl::optional<T> >
+    : absl::optional_internal::optional_hash_base<T> {};
 
 }  // namespace std
 
diff --git a/absl/types/optional_test.cc b/absl/types/optional_test.cc
index 25b44b17..65f43871 100644
--- a/absl/types/optional_test.cc
+++ b/absl/types/optional_test.cc
@@ -24,6 +24,17 @@
 #include "absl/meta/type_traits.h"
 #include "absl/strings/string_view.h"
 
+struct Hashable {};
+
+namespace std {
+template <>
+struct hash<Hashable> {
+  size_t operator()(const Hashable&) { return 0; }
+};
+}  // namespace std
+
+struct NonHashable {};
+
 namespace {
 
 std::string TypeQuals(std::string&) { return "&"; }
@@ -1434,6 +1445,17 @@ TEST(optionalTest, ValueType) {
       (std::is_same<absl::optional<int>::value_type, absl::nullopt_t>::value));
 }
 
+template <typename T>
+struct is_hash_enabled_for {
+  template <typename U, typename = decltype(std::hash<U>()(std::declval<U>()))>
+  static std::true_type test(int);
+
+  template <typename U>
+  static std::false_type test(...);
+
+  static constexpr bool value = decltype(test<T>(0))::value;
+};
+
 TEST(optionalTest, Hash) {
   std::hash<absl::optional<int>> hash;
   std::set<size_t> hashcodes;
@@ -1442,6 +1464,34 @@ TEST(optionalTest, Hash) {
     hashcodes.insert(hash(i));
   }
   EXPECT_GT(hashcodes.size(), 90);
+
+  static_assert(is_hash_enabled_for<absl::optional<int>>::value, "");
+  static_assert(is_hash_enabled_for<absl::optional<Hashable>>::value, "");
+
+#if defined(_MSC_VER) || (defined(_LIBCPP_VERSION) && \
+                          _LIBCPP_VERSION < 4000 && _LIBCPP_STD_VER > 11)
+  // For MSVC and libc++ (< 4.0 and c++14), std::hash primary template has a
+  // static_assert to catch any user-defined type that doesn't provide a hash
+  // specialization. So instantiating std::hash<absl::optional<T>> will result
+  // in a hard error which is not SFINAE friendly.
+#define ABSL_STD_HASH_NOT_SFINAE_FRIENDLY 1
+#endif
+
+#ifndef ABSL_STD_HASH_NOT_SFINAE_FRIENDLY
+  static_assert(!is_hash_enabled_for<absl::optional<NonHashable>>::value, "");
+#endif
+
+  // libstdc++ std::optional is missing remove_const_t, i.e. it's using
+  // std::hash<T> rather than std::hash<std::remove_const_t<T>>.
+  // Reference: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82262
+#ifndef __GLIBCXX__
+  static_assert(is_hash_enabled_for<absl::optional<const int>>::value, "");
+  static_assert(is_hash_enabled_for<absl::optional<const Hashable>>::value, "");
+  std::hash<absl::optional<const int>> c_hash;
+  for (int i = 0; i < 100; ++i) {
+    EXPECT_EQ(hash(i), c_hash(i));
+  }
+#endif
 }
 
 struct MoveMeNoThrow {