[PATCH] libstdc++: use new built-in trait __is_reference

[Ken Matsui via Gcc-patches]

libstdc++-v3/ChangeLog:

* include/std/type_traits (is_reference): Use __is_reference built-in trait.
---
diff --git a/libstdc++-v3/include/std/type_traits
b/libstdc++-v3/include/std/type_traits
index 2bd607a8b8f..18408d8ceb6 100644
--- a/libstdc++-v3/include/std/type_traits
+++ b/libstdc++-v3/include/std/type_traits
@@ -639,6 +639,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   // Composite type categories.

   /// is_reference
+#if __has_builtin(__is_reference)
+  template<typename _Tp>
+    struct is_reference
+    : public integral_constant<bool, __is_reference(_Tp)>
+    { };
+#else
   template<typename _Tp>
     struct is_reference
     : public false_type
@@ -653,6 +659,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     struct is_reference<_Tp&&>
     : public true_type
     { };
+#endif

   /// is_arithmetic
   template<typename _Tp>