Re: [PATCH v8 4/6] libstdc++: use new built-in trait __is_function for std::is_function

[Jonathan Wakely via Gcc-patches]

On Sat, 8 Jul 2023 at 06:15, Ken Matsui via Libstdc++
<libstd...@gcc.gnu.org> wrote:
>
> 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...@gcc.gnu.org>
> ---
>  libstdc++-v3/include/std/type_traits | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
>
> diff --git a/libstdc++-v3/include/std/type_traits 
> b/libstdc++-v3/include/std/type_traits
> index 2a14df7e5f9..954b57518de 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 +3204,14 @@ 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);
> +#else
>  template <typename _Tp>
>    inline constexpr bool is_function_v = is_function<_Tp>::value;

This fallback could be:

template <typename _Tp>
   inline constexpr bool is_function_v = !is_const_v<const _Tp>;

That would avoid instantiating std::is_function and std::is_const and
std::integral_constant, which should be significant.