Re: [PATCH v5 6/6] libstdc++: make std::is_object dispatch to new built-in traits

["François Dumont" <frs.dumont@gmail.com>]

On 13/06/2023 00:22, Ken Matsui via Libstdc++ wrote:
> This patch gets std::is_object to dispatch to new built-in traits,
> __is_function, __is_reference, and __is_void.
>
> libstdc++-v3/ChangeLog:
> 	* include/std/type_traits (is_object): Use new built-in traits,
> 	__is_function, __is_reference, and __is_void.
> 	(__is_object): Define this built-in-like macro.
> 	(is_object_v): Use built-in traits through the build-in-like macro.
>
> Signed-off-by: Ken Matsui <kmatsui@cs.washington.edu>
> ---
>   libstdc++-v3/include/std/type_traits | 19 +++++++++++++++++++
>   1 file changed, 19 insertions(+)
>
> diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
> index 780fcc00135..93335f94385 100644
> --- a/libstdc++-v3/include/std/type_traits
> +++ b/libstdc++-v3/include/std/type_traits
> @@ -682,11 +682,23 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>       { };
>   
>     /// is_object
> +#if __has_builtin(__is_function) && __has_builtin(__is_reference) \
> +    && __has_builtin(__is_void)
> +
> +#define __is_object(_Tp) \
> +  (!(__is_function(_Tp) || __is_reference(_Tp) || __is_void(_Tp)))

Is this evaluation order random ? Are all those builtin functions 
performances equivalent ?

I would have felt that __is_void is the simplest/fastest cause only for 
'void' so would have put it first.

> +
> +  template<typename _Tp>
> +    struct is_object
> +    : public __bool_constant<__is_object(_Tp)>
> +    { };
> +#else
>     template<typename _Tp>
>       struct is_object
>       : public __not_<__or_<is_function<_Tp>, is_reference<_Tp>,
>                             is_void<_Tp>>>::type
>       { };
> +#endif
>   
>     template<typename>
>       struct is_member_pointer;
> @@ -3235,8 +3247,15 @@ template <typename _Tp>
>     inline constexpr bool is_arithmetic_v = is_arithmetic<_Tp>::value;
>   template <typename _Tp>
>     inline constexpr bool is_fundamental_v = is_fundamental<_Tp>::value;
> +
> +#ifdef __is_object
> +template <typename _Tp>
> +  inline constexpr bool is_object_v = __is_object(_Tp);
> +#else
>   template <typename _Tp>
>     inline constexpr bool is_object_v = is_object<_Tp>::value;
> +#endif
> +
>   template <typename _Tp>
>     inline constexpr bool is_scalar_v = is_scalar<_Tp>::value;
>   template <typename _Tp>