Export of internal Abseil changes

--
6b5be2524a088d0f4e8475794dc71232a24e94d8 by Abseil Team <absl-team@google.com>:

Enable ABSL_HAVE_ATTRIBUTE_WEAK for Windows with Clang >= 9.0.0

The bug (https://bugs.llvm.org/show_bug.cgi?id=37598) motivated the workaround
was fixed in 9.0.0.

PiperOrigin-RevId: 365682074

--
c16b7784978a370658dce6d82cb7055316a79bcc by Abseil Team <absl-team@google.com>:

Add IsFlat() evaluation to GetFlatAux for RingBuffer

PiperOrigin-RevId: 365666501

--
c064eb686a3c036e093e71126c45f97d3a921569 by Abseil Team <absl-team@google.com>:

Implement C++11 compatible std::remove_cvref added in C++20

PiperOrigin-RevId: 365606639

--
af2e7e055172da914e63c05308aedb68e197661e by Abseil Team <absl-team@google.com>:

Add IsFlat() support to CordRepRing

PiperOrigin-RevId: 365562090

--
2cfeff9280f4967c4f828812bfe153b4e9cbabb7 by Abseil Team <absl-team@google.com>:

Make unit test for TryFlat on 'substring of rep' explicit

PiperOrigin-RevId: 365081382
GitOrigin-RevId: 6b5be2524a088d0f4e8475794dc71232a24e94d8
Change-Id: Ibb577748176217ce237614a6fe77c05375a97003

2 files changed, 49 insertions, 0 deletions

diff --git a/absl/meta/type_traits.h b/absl/meta/type_traits.h
index d5cb5f3b..b5427a43 100644
--- a/absl/meta/type_traits.h
+++ b/absl/meta/type_traits.h
@@ -499,6 +499,27 @@ struct is_trivially_copy_assignable
 #endif  // ABSL_HAVE_STD_IS_TRIVIALLY_ASSIGNABLE
 };
 
+#if defined(__cpp_lib_remove_cvref) && __cpp_lib_remove_cvref >= 201711L
+template <typename T>
+using remove_cvref = std::remove_cvref<T>;
+
+template <typename T>
+using remove_cvref_t = typename std::remove_cvref<T>::type;
+#else
+// remove_cvref()
+//
+// C++11 compatible implementation of std::remove_cvref which was added in
+// C++20.
+template <typename T>
+struct remove_cvref {
+  using type =
+      typename std::remove_cv<typename std::remove_reference<T>::type>::type;
+};
+
+template <typename T>
+using remove_cvref_t = typename remove_cvref<T>::type;
+#endif
+
 namespace type_traits_internal {
 // is_trivially_copyable()
 //
diff --git a/absl/meta/type_traits_test.cc b/absl/meta/type_traits_test.cc
index 1aafd0d4..0ef5b665 100644
--- a/absl/meta/type_traits_test.cc
+++ b/absl/meta/type_traits_test.cc
@@ -942,6 +942,34 @@ TEST(TypeTraitsTest, TestTriviallyCopyable) {
       absl::type_traits_internal::is_trivially_copyable<Trivial&>::value);
 }
 
+TEST(TypeTraitsTest, TestRemoveCVRef) {
+  EXPECT_TRUE(
+      (std::is_same<typename absl::remove_cvref<int>::type, int>::value));
+  EXPECT_TRUE(
+      (std::is_same<typename absl::remove_cvref<int&>::type, int>::value));
+  EXPECT_TRUE(
+      (std::is_same<typename absl::remove_cvref<int&&>::type, int>::value));
+  EXPECT_TRUE((
+      std::is_same<typename absl::remove_cvref<const int&>::type, int>::value));
+  EXPECT_TRUE(
+      (std::is_same<typename absl::remove_cvref<int*>::type, int*>::value));
+  // Does not remove const in this case.
+  EXPECT_TRUE((std::is_same<typename absl::remove_cvref<const int*>::type,
+                            const int*>::value));
+  EXPECT_TRUE((std::is_same<typename absl::remove_cvref<int[2]>::type,
+                            int[2]>::value));
+  EXPECT_TRUE((std::is_same<typename absl::remove_cvref<int(&)[2]>::type,
+                            int[2]>::value));
+  EXPECT_TRUE((std::is_same<typename absl::remove_cvref<int(&&)[2]>::type,
+                            int[2]>::value));
+  EXPECT_TRUE((std::is_same<typename absl::remove_cvref<const int[2]>::type,
+                            int[2]>::value));
+  EXPECT_TRUE((std::is_same<typename absl::remove_cvref<const int(&)[2]>::type,
+                            int[2]>::value));
+  EXPECT_TRUE((std::is_same<typename absl::remove_cvref<const int(&&)[2]>::type,
+                            int[2]>::value));
+}
+
 #define ABSL_INTERNAL_EXPECT_ALIAS_EQUIVALENCE(trait_name, ...)          \
   EXPECT_TRUE((std::is_same<typename std::trait_name<__VA_ARGS__>::type, \
                             absl::trait_name##_t<__VA_ARGS__>>::value))