[PATCH v4 4/6] libstdc++: use new built-in trait __is_function for std::is_function

[Ken Matsui via Gcc-patches]

This patch gets std::is_function to dispatch to new built-in trait
__is_function.

libstdc++-v3/ChangeLog:

        * include/std/type_traits (is_function): Use __is_function built-in
        trait.
        (is_function_v): Likewise.

Signed-off-by: Ken Matsui <kmat...@cs.washington.edu>
---
 libstdc++-v3/include/std/type_traits | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/std/type_traits 
b/libstdc++-v3/include/std/type_traits
index b2eb4bd3e7f..c0fa748da2e 100644
--- a/libstdc++-v3/include/std/type_traits
+++ b/libstdc++-v3/include/std/type_traits
@@ -594,6 +594,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     { };
 
   /// is_function
+#if __has_builtin(__is_function)
+  template<typename _Tp>
+    struct is_function
+    : public __bool_constant<__is_function(_Tp)>
+    { };
+#else
   template<typename _Tp>
     struct is_function
     : public __bool_constant<!is_const<const _Tp>::value> { };
@@ -605,6 +611,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template<typename _Tp>
     struct is_function<_Tp&&>
     : public false_type { };
+#endif
 
 #define __cpp_lib_is_null_pointer 201309L
 
@@ -3198,7 +3205,7 @@ template <typename _Tp>
 template <typename _Tp>
   inline constexpr bool is_class_v = __is_class(_Tp);
 template <typename _Tp>
-  inline constexpr bool is_function_v = is_function<_Tp>::value;
+  inline constexpr bool is_function_v = __is_function(_Tp);
 template <typename _Tp>
   inline constexpr bool is_reference_v = __is_reference(_Tp);
 template <typename _Tp>
-- 
2.41.0