From b6a1039bfcd6163b923c84e23e11439f42c98bc7 Mon Sep 17 00:00:00 2001
From: Abseil Team <absl-team@google.com>
Date: Fri, 24 Mar 2023 17:59:28 -0700
Subject: type_traits: provide a better fallback for is_trivially_relocatable.

We can do a lot better than always saying "no" on platforms without the
__is_trivially_relocatable builtin. This will allow using this type trait from
the move constructors of InlinedVector in a future CL, without needing to open
code the fallback logic.

PiperOrigin-RevId: 519281125
Change-Id: I0d55f019331966f58074850d6f77c7eab49f2c53
---
 absl/meta/type_traits.h       | 10 +++++++++-
 absl/meta/type_traits_test.cc | 17 +++++++++++++++++
 2 files changed, 26 insertions(+), 1 deletion(-)

(limited to 'absl/meta')

diff --git a/absl/meta/type_traits.h b/absl/meta/type_traits.h
index 25462580..f713b78c 100644
--- a/absl/meta/type_traits.h
+++ b/absl/meta/type_traits.h
@@ -497,12 +497,20 @@ using swap_internal::StdSwapIsUnconstrained;
 // https://clang.llvm.org/docs/LanguageExtensions.html#:~:text=__is_trivially_relocatable
 //
 #if ABSL_HAVE_BUILTIN(__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]].
 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 trivally relocatable.
 template <class T>
-struct is_trivially_relocatable : std::integral_constant<bool, false> {};
+struct is_trivially_relocatable
+    : absl::conjunction<absl::is_trivially_move_constructible<T>,
+                        absl::is_trivially_destructible<T>> {};
 #endif
 
 // absl::is_constant_evaluated()
diff --git a/absl/meta/type_traits_test.cc b/absl/meta/type_traits_test.cc
index 3e7cfbe7..d9e36887 100644
--- a/absl/meta/type_traits_test.cc
+++ b/absl/meta/type_traits_test.cc
@@ -743,6 +743,23 @@ TEST(TypeTraitsTest, IsNothrowSwappable) {
   EXPECT_TRUE(IsNothrowSwappable<adl_namespace::SpecialNoexceptSwap>::value);
 }
 
+TEST(TriviallyRelocatable, PrimitiveTypes) {
+  static_assert(absl::is_trivially_relocatable<int>::value, "");
+  static_assert(absl::is_trivially_relocatable<char>::value, "");
+  static_assert(absl::is_trivially_relocatable<void*>::value, "");
+}
+
+// User-defined types can be trivially relocatable as long as they don't have a
+// user-provided move constructor or destructor.
+TEST(TriviallyRelocatable, UserDefinedTriviallyReconstructible) {
+  struct S {
+    int x;
+    int y;
+  };
+
+  static_assert(absl::is_trivially_relocatable<S>::value, "");
+}
+
 // A user-provided move constructor disqualifies a type from being trivially
 // relocatable.
 TEST(TriviallyRelocatable, UserProvidedMoveConstructor) {
-- 
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