Change the definition of is_trivially_relocatable to be a bit less conservative.

Instead of ignoring __is_trivially_relocatable, use __is_trivially_relocatable && std::is_trivially_move_assignable.

PiperOrigin-RevId: 709106861
Change-Id: I2d53bd440133ad17bb676e53ee6a17d420a565d1

Author: ezbr

absl/meta/type_traits.h

@@ -503,10 +503,11 @@ using swap_internal::Swap;
 // remove the condition.
 //
 // Clang on all platforms fails to detect that a type with a user-provided
-// move-assignment operator is not trivially relocatable. So in fact we
-// opt out of Clang altogether, for now.
+// move-assignment operator is not trivially relocatable so we also check for
+// is_trivially_move_assignable for Clang.
 //
-// TODO(b/325479096): Remove the opt-out once Clang's behavior is fixed.
+// TODO(b/325479096): Remove the Clang is_trivially_move_assignable version once
+// Clang's behavior is fixed.
 //
 // According to https://github.com/abseil/abseil-cpp/issues/1479, this does not
 // work with NVCC either.
@@ -516,6 +517,15 @@ using swap_internal::Swap;
 template <class T>
 struct is_trivially_relocatable
     : std::integral_constant<bool, __is_trivially_relocatable(T)> {};
+#elif ABSL_HAVE_BUILTIN(__is_trivially_relocatable) && defined(__clang__) && \
+    !(defined(_WIN32) || defined(_WIN64)) && !defined(__APPLE__) &&          \
+    !defined(__NVCC__)
+template <class T>
+struct is_trivially_relocatable
+    : std::integral_constant<
+          bool, std::is_trivially_copyable<T>::value ||
+                    (__is_trivially_relocatable(T) &&
+                     std::is_trivially_move_assignable<T>::value)> {};
 #else
 // Otherwise we use a fallback that detects only those types we can feasibly
 // detect. Any type that is trivially copyable is by definition trivially

absl/meta/type_traits_test.cc

@@ -769,6 +769,8 @@ TEST(TriviallyRelocatable, UserProvidedDestructor) {
 // __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) ||    \
@@ -779,8 +781,28 @@ TEST(TriviallyRelocatable, TrivialAbi) {
   struct ABSL_ATTRIBUTE_TRIVIAL_ABI S {
     S(S&&) {}       // NOLINT(modernize-use-equals-default)
     S(const S&) {}  // NOLINT(modernize-use-equals-default)
-    void operator=(S&&) {}
-    void operator=(const S&) {}
+    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)
   };