[PATCH v3 4/6] libstdc++: use new built-in trait __is_function for std::is_function

[Patrick Palka]

On Sun, 2 Apr 2023, Ken Matsui via Gcc-patches wrote:

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

For std::is_function and other predicate-like type traits, I think we also
want to make the corresponding variable template is_function_v directly
use the built-in too.

> 
> libstdc++-v3/ChangeLog:
> 
> 	* include/std/type_traits (is_function): Use __is_function built-in
> 	trait.
> 
> Signed-off-by: Ken Matsui <kmatsui@cs.washington.edu>
> ---
>  libstdc++-v3/include/std/type_traits | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
> index 58a732735c8..9eafd6b16f2 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
>  
> -- 
> 2.40.0
> 
>