type_traits: don't use __is_trivially_relocatable with Clang on Windows.

It currently gives the wrong result for types with a user-provided destructor:

    struct S {
      ~S();
    };

    static_assert(!__is_trivially_relocatable(S));

PiperOrigin-RevId: 519855600
Change-Id: I5f0659ad0831974805a5ed4b615e3b6250d23154

absl/meta/type_traits.h

@@ -495,11 +495,18 @@ using swap_internal::StdSwapIsUnconstrained;
 // Upstream documentation:
 //
 // 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]].
+//
+// 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
+// there.
+//
+// TODO(b/275003464): remove the opt-out once the bug is fixed.
+#if ABSL_HAVE_BUILTIN(__is_trivially_relocatable) && \
+    !(defined(__clang__) && (defined(_WIN32) || defined(_WIN64)))
 template <class T>
 struct is_trivially_relocatable
     : std::integral_constant<bool, __is_trivially_relocatable(T)> {};

absl/meta/type_traits_test.cc

@@ -780,20 +780,6 @@ TEST(TriviallyRelocatable, UserProvidedCopyConstructor) {
   static_assert(!absl::is_trivially_relocatable<S>::value, "");
 }
 
-// HACK: disable this test on Windows, which includes lexan, which gives the
-// incorrect result for this case (http://b/275003464).
-//
-// TODO(b/275003464): make this hack more precise after figuring out how to
-// detect --config=lexan in particular. If there is no reliable way, modify
-// --config=lexan itself to set a define we can use.
-//
-// TODO(b/274984172): hoist this hack to the definition of
-// absl::is_trivially_relocatable itself, since it's currently giving the wrong
-// results under lexan. We can't trust __is_trivially_relocatable on that
-// platform.
-//
-// TODO(b/275003464): remove this hack when the bug is fixed.
-#if !(defined(_WIN32) || defined(_WIN64))
 // A user-provided destructor disqualifies a type from being trivially
 // relocatable.
 TEST(TriviallyRelocatable, UserProvidedDestructor) {
@@ -803,10 +789,12 @@ TEST(TriviallyRelocatable, UserProvidedDestructor) {
 
   static_assert(!absl::is_trivially_relocatable<S>::value, "");
 }
-#endif
 
-#if defined(ABSL_HAVE_ATTRIBUTE_TRIVIAL_ABI) && \
-    ABSL_HAVE_BUILTIN(__is_trivially_relocatable)
+// TODO(b/275003464): remove the opt-out for Clang on Windows once
+// __is_trivially_relocatable is used there again.
+#if defined(ABSL_HAVE_ATTRIBUTE_TRIVIAL_ABI) &&      \
+    ABSL_HAVE_BUILTIN(__is_trivially_relocatable) && \
+    !(defined(__clang__) && (defined(_WIN32) || defined(_WIN64)))
 // 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.