[PATCH v10 4/5] 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. Optimize its implementation.

Signed-off-by: Ken Matsui <kmat...@gcc.gnu.org>
---
 libstdc++-v3/include/std/type_traits | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/std/type_traits 
b/libstdc++-v3/include/std/type_traits
index 2a14df7e5f9..7ef50a2e64f 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
 
@@ -3197,8 +3204,18 @@ template <typename _Tp>
   inline constexpr bool is_union_v = __is_union(_Tp);
 template <typename _Tp>
   inline constexpr bool is_class_v = __is_class(_Tp);
+
+#if __has_builtin(__is_function)
 template <typename _Tp>
-  inline constexpr bool is_function_v = is_function<_Tp>::value;
+  inline constexpr bool is_function_v = __is_function(_Tp);
+#else
+template <typename _Tp>
+  inline constexpr bool is_function_v = !is_const_v<const _Tp>;
+template <typename _Tp>
+  inline constexpr bool is_function_v<_Tp&> = false;
+template <typename _Tp>
+  inline constexpr bool is_function_v<_Tp&&> = false;
+#endif
 
 #if __has_builtin(__is_reference)
 template <typename _Tp>
-- 
2.41.0