Only use GCC-specific __is_same_as built-in conditionally

Clang doesn't support __is_same_as but provides __is_same instead.
Restore the original implementation (pre r276891) when neither of those
built-ins is available.

	* include/bits/c++config (_GLIBCXX_BUILTIN_IS_SAME_AS): Define to
	one of __is_same_as or __is_same when available.
	* include/std/concepts (__detail::__same_as): Use std::is_same_v.
	* include/std/type_traits (is_same) [_GLIBCXX_BUILTIN_IS_SAME_AS]:
	Use new macro instead of __is_same_as.
	(is_same) [!_GLIBCXX_BUILTIN_IS_SAME_AS]: Restore partial
	specialization.
	(is_same_v) [_GLIBCXX_BUILTIN_IS_SAME_AS]: Use new macro.
	(is_same_v) [!_GLIBCXX_BUILTIN_IS_SAME_AS]: Use std::is_same.

From-SVN: r277058

libstdc++-v3/ChangeLog

+2019-10-16  Jonathan Wakely  <jwakely@redhat.com>
+
+	* include/bits/c++config (_GLIBCXX_BUILTIN_IS_SAME_AS): Define to
+	one of __is_same_as or __is_same when available.
+	* include/std/concepts (__detail::__same_as): Use std::is_same_v.
+	* include/std/type_traits (is_same) [_GLIBCXX_BUILTIN_IS_SAME_AS]:
+	Use new macro instead of __is_same_as.
+	(is_same) [!_GLIBCXX_BUILTIN_IS_SAME_AS]: Restore partial
+	specialization.
+	(is_same_v) [_GLIBCXX_BUILTIN_IS_SAME_AS]: Use new macro.
+	(is_same_v) [!_GLIBCXX_BUILTIN_IS_SAME_AS]: Use std::is_same.
+
 2019-10-16  François Dumont  <fdumont@gcc.gnu.org>
 
 	* src/c++11/debug.cc (print_field): Replace constness_names <unknown>

libstdc++-v3/include/bits/c++config

@@ -633,6 +633,7 @@ namespace std
 # define _GLIBCXX_HAVE_BUILTIN_HAS_UNIQ_OBJ_REP 1
 # define _GLIBCXX_HAVE_BUILTIN_IS_AGGREGATE 1
 # define _GLIBCXX_HAVE_BUILTIN_LAUNDER 1
+# define _GLIBCXX_BUILTIN_IS_SAME_AS(T, U) __is_same_as(T, U)
 # if __GNUC__ >= 9
 #  define _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED 1
 # endif
@@ -650,6 +651,9 @@ namespace std
 # if __has_builtin(__builtin_is_constant_evaluated)
 #  define _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED 1
 # endif
+# if ! __is_identifier(__is_same)
+#  define _GLIBCXX_BUILTIN_IS_SAME_AS(T, U) __is_same(T, U)
+# endif
 #endif // GCC
 
 // PSTL configuration

libstdc++-v3/include/std/concepts

@@ -23,7 +23,7 @@
 // <http://www.gnu.org/licenses/>.
 
 /** @file include/concepts
- *  This is __a Standard C++ Library header.
+ *  This is a Standard C++ Library header.
  *  @ingroup concepts
  */
 
@@ -54,7 +54,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   namespace __detail
   {
     template<typename _Tp, typename _Up>
-      concept __same_as = __is_same_as(_Tp, _Up);
+      concept __same_as = std::is_same_v<_Tp, _Up>;
   } // namespace __detail
 
   /// [concept.same], concept same_as

libstdc++-v3/include/std/type_traits

@@ -1390,9 +1390,20 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   /// is_same
   template<typename _Tp, typename _Up>
     struct is_same
-    : public integral_constant<bool, __is_same_as(_Tp, _Up)>
+#ifdef _GLIBCXX_BUILTIN_IS_SAME_AS
+    : public integral_constant<bool, _GLIBCXX_BUILTIN_IS_SAME_AS(_Tp, _Up)>
+#else
+    : public false_type
+#endif
     { };
 
+#ifndef _GLIBCXX_BUILTIN_IS_SAME_AS
+  template<typename _Tp>
+    struct is_same<_Tp, _Tp>
+    : public true_type
+    { };
+#endif
+
   /// is_base_of
   template<typename _Base, typename _Derived>
     struct is_base_of
@@ -3154,8 +3165,13 @@ template <typename _Tp>
   inline constexpr size_t rank_v = rank<_Tp>::value;
 template <typename _Tp, unsigned _Idx = 0>
   inline constexpr size_t extent_v = extent<_Tp, _Idx>::value;
+#ifdef _GLIBCXX_BUILTIN_IS_SAME_AS
 template <typename _Tp, typename _Up>
-  inline constexpr bool is_same_v = __is_same_as(_Tp, _Up);
+  inline constexpr bool is_same_v = _GLIBCXX_BUILTIN_IS_SAME_AS(_Tp, _Up);
+#else
+template <typename _Tp, typename _Up>
+  inline constexpr bool is_same_v = std::is_same<_Tp, _Up>::value;
+#endif
 template <typename _Base, typename _Derived>
   inline constexpr bool is_base_of_v = is_base_of<_Base, _Derived>::value;
 template <typename _From, typename _To>