Use __builtin_is_cpp_trivially_relocatable to implement

absl::is_trivially_relocatable in a way that is compatible with PR2786
in the upcoming C++26.

https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p2786r11.html

This change is being made now because Chromium is reporting that a
recent LLVM commit adds deprecation warnings for
__is_trivially_relocatable.
llvm/llvm-project#138835

PiperOrigin-RevId: 756408712
Change-Id: Iacf966ed2ebfd436d52d180f0dab34465b3c7176

Author: derekmauro

absl/meta/type_traits.h

@@ -324,11 +324,17 @@ using swap_internal::Swap;
 
 // absl::is_trivially_relocatable<T>
 //
+// https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p2786r11.html
+//
 // Detects whether a type is known to be "trivially relocatable" -- meaning it
 // can be relocated from one place to another as if by memcpy/memmove.
 // This implies that its object representation doesn't depend on its address,
 // and also none of its special member functions do anything strange.
 //
+// Note that when relocating the caller code should ensure that if the object is
+// polymorphic, the dynamic type is of the most derived type. Padding bytes
+// should not be copied.
+//
 // This trait is conservative. If it's true then the type is definitely
 // trivially relocatable, but if it's false then the type may or may not be. For
 // example, std::vector<int> is trivially relocatable on every known STL
@@ -346,11 +352,7 @@ using swap_internal::Swap;
 //
 // Upstream documentation:
 //
-// https://clang.llvm.org/docs/LanguageExtensions.html#:~:text=__is_trivially_relocatable
-
-// If the compiler offers a builtin that tells us the answer, we can use that.
-// This covers all of the cases in the fallback below, plus types that opt in
-// using e.g. [[clang::trivial_abi]].
+// https://clang.llvm.org/docs/LanguageExtensions.html#:~:text=__builtin_is_cpp_trivially_relocatable
 //
 // Clang on Windows has the builtin, but it falsely claims types with a
 // user-provided destructor are trivial (http://b/275003464). So we opt out
@@ -375,15 +377,22 @@ using swap_internal::Swap;
 //
 // According to https://github.com/abseil/abseil-cpp/issues/1479, this does not
 // work with NVCC either.
-#if ABSL_HAVE_BUILTIN(__is_trivially_relocatable) && \
-    (defined(__cpp_impl_trivially_relocatable) ||    \
-     (!defined(__clang__) && !defined(__APPLE__) && !defined(__NVCC__)))
+#if ABSL_HAVE_BUILTIN(__builtin_is_cpp_trivially_relocatable)
+// https://github.com/llvm/llvm-project/pull/127636#pullrequestreview-2637005293
+// In the current implementation, __builtin_is_cpp_trivially_relocatable will
+// only return true for types that are trivially relocatable according to the
+// standard. Notably, this means that marking a type [[clang::trivial_abi]] aka
+// ABSL_HAVE_ATTRIBUTE_TRIVIAL_ABI will have no effect on this trait.
 template <class T>
 struct is_trivially_relocatable
-    : std::integral_constant<bool, __is_trivially_relocatable(T)> {};
+    : std::integral_constant<bool, __builtin_is_cpp_trivially_relocatable(T)> {
+};
 #elif ABSL_HAVE_BUILTIN(__is_trivially_relocatable) && defined(__clang__) && \
     !(defined(_WIN32) || defined(_WIN64)) && !defined(__APPLE__) &&          \
     !defined(__NVCC__)
+// https://github.com/llvm/llvm-project/pull/139061
+//  __is_trivially_relocatable is deprecated.
+// TODO(b/325479096): Remove this case.
 template <class T>
 struct is_trivially_relocatable
     : std::integral_constant<

absl/meta/type_traits_test.cc

@@ -333,51 +333,6 @@ TEST(TriviallyRelocatable, UserProvidedDestructor) {
   static_assert(!absl::is_trivially_relocatable<S>::value, "");
 }
 
-// TODO(b/275003464): remove the opt-out for Clang on Windows once
-// __is_trivially_relocatable is used there again.
-// TODO(b/324278148): remove the opt-out for Apple once
-// __is_trivially_relocatable is fixed there.
-// TODO(b/325479096): remove the opt-out for Clang once
-// __is_trivially_relocatable is fixed there.
-#if defined(ABSL_HAVE_ATTRIBUTE_TRIVIAL_ABI) &&      \
-    ABSL_HAVE_BUILTIN(__is_trivially_relocatable) && \
-    (defined(__cpp_impl_trivially_relocatable) ||    \
-     (!defined(__clang__) && !defined(__APPLE__) && !defined(__NVCC__)))
-// A type marked with the "trivial ABI" attribute is trivially relocatable even
-// if it has user-provided special members.
-TEST(TriviallyRelocatable, TrivialAbi) {
-  struct ABSL_ATTRIBUTE_TRIVIAL_ABI S {
-    S(S&&) {}       // NOLINT(modernize-use-equals-default)
-    S(const S&) {}  // NOLINT(modernize-use-equals-default)
-    S& operator=(S&&) { return *this; }
-    S& operator=(const S&) { return *this; }
-    ~S() {}  // NOLINT(modernize-use-equals-default)
-  };
-
-  static_assert(absl::is_trivially_relocatable<S>::value, "");
-}
-#endif
-
-// TODO(b/275003464): remove the opt-out for Clang on Windows once
-// __is_trivially_relocatable is used there again.
-// TODO(b/324278148): remove the opt-out for Apple once
-// __is_trivially_relocatable is fixed there.
-#if defined(ABSL_HAVE_ATTRIBUTE_TRIVIAL_ABI) &&                            \
-    ABSL_HAVE_BUILTIN(__is_trivially_relocatable) && defined(__clang__) && \
-    !(defined(_WIN32) || defined(_WIN64)) && !defined(__APPLE__) &&        \
-    !defined(__NVCC__)
-// A type marked with the "trivial ABI" attribute is trivially relocatable even
-// if it has a user-provided copy constructor and a user-provided destructor.
-TEST(TriviallyRelocatable, TrivialAbi_NoUserProvidedMove) {
-  struct ABSL_ATTRIBUTE_TRIVIAL_ABI S {
-    S(const S&) {}  // NOLINT(modernize-use-equals-default)
-    ~S() {}  // NOLINT(modernize-use-equals-default)
-  };
-
-  static_assert(absl::is_trivially_relocatable<S>::value, "");
-}
-#endif
-
 #ifdef ABSL_HAVE_CONSTANT_EVALUATED
 
 constexpr int64_t NegateIfConstantEvaluated(int64_t i) {