absl::is_trivially_relocatable now respects assignment operators

absl/meta/type_traits.h

@@ -471,12 +471,17 @@ using swap_internal::StdSwapIsUnconstrained;
 // absl::is_trivially_relocatable<T>
 //
 // Detects whether a type is known to be "trivially relocatable" -- meaning it
-// can be relocated without invoking the constructor/destructor, using a form of
-// move elision.
-//
-// This trait is conservative, for backwards compatibility. If it's true then
-// the type is definitely trivially relocatable, but if it's false then the type
-// may or may not be.
+// 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.
+//
+// This trait is conservative. If it's true then the type is definitely
+// trivially relocatable, but there are many types which are "Platonically"
+// trivially relocatable but for which the type trait returns false because
+// it can't introspect into the special members and see that they're not
+// doing anything strange. (For example, std::vector<int> is trivially
+// relocatable on every known STL implementation, but
+// absl::is_trivially_relocatable<std::vector<int>> remains false.)
 //
 // Example:
 //
@@ -509,22 +514,27 @@ using swap_internal::StdSwapIsUnconstrained;
 // TODO(b/324278148): If all versions we use have the bug fixed, then
 // 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.
+//
+// TODO: remove the opt-out once Clang's behavior is fixed.
+//
 // 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(__clang__) && (defined(_WIN32) || defined(_WIN64))) && \
-    !(defined(__APPLE__)) && !defined(__NVCC__)
+#if ABSL_HAVE_BUILTIN(__is_trivially_relocatable) && \
+    (defined(__cpp_impl_trivially_relocatable) ||    \
+     (!defined(__clang__) && !defined(__APPLE__) && !defined(__NVCC__)))
 template <class T>
 struct is_trivially_relocatable
     : std::integral_constant<bool, __is_trivially_relocatable(T)> {};
 #else
 // Otherwise we use a fallback that detects only those types we can feasibly
-// detect. Any time that has trivial move-construction and destruction
-// operations is by definition trivially relocatable.
+// detect. Any type that is trivially copyable is by definition trivially
+// relocatable.
 template <class T>
 struct is_trivially_relocatable
-    : absl::conjunction<absl::is_trivially_move_constructible<T>,
-                        absl::is_trivially_destructible<T>> {};
+    : std::is_trivially_copyable<T> {};
 #endif
 
 // absl::is_constant_evaluated()

absl/meta/type_traits_test.cc

@@ -751,7 +751,7 @@ TEST(TriviallyRelocatable, PrimitiveTypes) {
 
 // User-defined types can be trivially relocatable as long as they don't have a
 // user-provided move constructor or destructor.
-TEST(TriviallyRelocatable, UserDefinedTriviallyReconstructible) {
+TEST(TriviallyRelocatable, UserDefinedTriviallyRelocatable) {
   struct S {
     int x;
     int y;
@@ -780,6 +780,28 @@ TEST(TriviallyRelocatable, UserProvidedCopyConstructor) {
   static_assert(!absl::is_trivially_relocatable<S>::value, "");
 }
 
+// A user-provided copy assignment operator disqualifies a type from
+// being trivially relocatable.
+TEST(TriviallyRelocatable, UserProvidedCopyAssignment) {
+  struct S {
+    S(const S&) = default;
+    S& operator=(const S&) { return *this; }  // NOLINT(modernize-use-equals-default)
+  };
+
+  static_assert(!absl::is_trivially_relocatable<S>::value, "");
+}
+
+// A user-provided move assignment operator disqualifies a type from
+// being trivially relocatable.
+TEST(TriviallyRelocatable, UserProvidedMoveAssignment) {
+  struct S {
+    S(S&&) = default;
+    S& operator=(S&&) { return *this; }  // NOLINT(modernize-use-equals-default)
+  };
+
+  static_assert(!absl::is_trivially_relocatable<S>::value, "");
+}
+
 // A user-provided destructor disqualifies a type from being trivially
 // relocatable.
 TEST(TriviallyRelocatable, UserProvidedDestructor) {
@@ -796,15 +818,16 @@ TEST(TriviallyRelocatable, UserProvidedDestructor) {
 // __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(__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 move/copy constructors and a user-provided
-// destructor.
-TEST(TrivallyRelocatable, TrivialAbi) {
+// 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)
+    void operator=(S&&) {}
+    void operator=(const S&) {}
     ~S() {}         // NOLINT(modernize-use-equals-default)
   };
 
@@ -824,7 +847,7 @@ constexpr int64_t NegateIfConstantEvaluated(int64_t i) {
 
 #endif  // ABSL_HAVE_CONSTANT_EVALUATED
 
-TEST(TrivallyRelocatable, is_constant_evaluated) {
+TEST(IsConstantEvaluated, is_constant_evaluated) {
 #ifdef ABSL_HAVE_CONSTANT_EVALUATED
   constexpr int64_t constant = NegateIfConstantEvaluated(42);
   EXPECT_EQ(constant, -42);