[PATCH v4 2/6] libstdc++: use new built-in trait __is_reference for std::is_reference

[Ken Matsui via Gcc-patches]

This patch gets std::is_reference to dispatch to new built-in trait
__is_reference.

libstdc++-v3/ChangeLog:

        * include/std/type_traits (is_reference): Use __is_reference built-in
        trait.
        (is_reference_v): Likewise.

Signed-off-by: Ken Matsui <kmat...@cs.washington.edu>
---
 libstdc++-v3/include/std/type_traits | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/libstdc++-v3/include/std/type_traits 
b/libstdc++-v3/include/std/type_traits
index 0e7a9c9c7f3..b2eb4bd3e7f 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 __bool_constant<__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>
@@ -3193,11 +3200,7 @@ template <typename _Tp>
 template <typename _Tp>
   inline constexpr bool is_function_v = is_function<_Tp>::value;
 template <typename _Tp>
-  inline constexpr bool is_reference_v = false;
-template <typename _Tp>
-  inline constexpr bool is_reference_v<_Tp&> = true;
-template <typename _Tp>
-  inline constexpr bool is_reference_v<_Tp&&> = true;
+  inline constexpr bool is_reference_v = __is_reference(_Tp);
 template <typename _Tp>
   inline constexpr bool is_arithmetic_v = is_arithmetic<_Tp>::value;
 template <typename _Tp>
-- 
2.41.0