On Mon, Oct 13, 2025 at 6:42 AM Patrick Palka <[email protected]> wrote:
> ... and in passing use requires-clauses instead of void_t based SFINAE.
> This is a non-functional change that'll simplify implementing the
> P2655R3 change to common_reference.
>
> PR c++/120446
>
> libstdc++-v3/ChangeLog:
>
> * include/std/type_traits (__common_reference_impl): Rewrite
> partial specializations to use requires-clause instead of
> an additional void_t template parameter. Consolidate the
> partial specializations corresponding to bullet 1.
> ---
>
LGTM, I believe nothing changed.
> libstdc++-v3/include/std/type_traits | 38 +++++++++-------------------
> 1 file changed, 12 insertions(+), 26 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/type_traits
> b/libstdc++-v3/include/std/type_traits
> index 77ebb7e2c2f9..8b5110464e50 100644
> --- a/libstdc++-v3/include/std/type_traits
> +++ b/libstdc++-v3/include/std/type_traits
> @@ -4208,7 +4208,7 @@ template<typename _Ret, typename _Fn, typename...
> _Args>
> { using type = _Tp0; };
>
> /// @cond undocumented
> - template<typename _Tp1, typename _Tp2, int _Bullet = 1, typename = void>
> + template<typename _Tp1, typename _Tp2, int _Bullet = 1>
> struct __common_reference_impl
> : __common_reference_impl<_Tp1, _Tp2, _Bullet + 1>
> { };
> @@ -4221,46 +4221,32 @@ template<typename _Ret, typename _Fn, typename...
> _Args>
>
> // If T1 and T2 are reference types and COMMON-REF(T1, T2) is
> well-formed, ...
> template<typename _Tp1, typename _Tp2>
> - struct __common_reference_impl<_Tp1&, _Tp2&, 1,
> - void_t<__common_ref<_Tp1&, _Tp2&>>>
> - { using type = __common_ref<_Tp1&, _Tp2&>; };
> -
> - template<typename _Tp1, typename _Tp2>
> - struct __common_reference_impl<_Tp1&&, _Tp2&&, 1,
> - void_t<__common_ref<_Tp1&&, _Tp2&&>>>
> - { using type = __common_ref<_Tp1&&, _Tp2&&>; };
> -
> - template<typename _Tp1, typename _Tp2>
> - struct __common_reference_impl<_Tp1&, _Tp2&&, 1,
> - void_t<__common_ref<_Tp1&, _Tp2&&>>>
> - { using type = __common_ref<_Tp1&, _Tp2&&>; };
> -
> - template<typename _Tp1, typename _Tp2>
> - struct __common_reference_impl<_Tp1&&, _Tp2&, 1,
> - void_t<__common_ref<_Tp1&&, _Tp2&>>>
> - { using type = __common_ref<_Tp1&&, _Tp2&>; };
> + requires is_reference_v<_Tp1> && is_reference_v<_Tp2>
> + && requires { typename __common_ref<_Tp1, _Tp2>; }
> + struct __common_reference_impl<_Tp1, _Tp2, 1>
> + { using type = __common_ref<_Tp1, _Tp2>; };
>
> // Otherwise, if basic_common_reference<...>::type is well-formed, ...
> template<typename _Tp1, typename _Tp2>
> - struct __common_reference_impl<_Tp1, _Tp2, 2,
> - void_t<__basic_common_ref<_Tp1, _Tp2>>>
> + requires requires { typename __basic_common_ref<_Tp1, _Tp2>; }
> + struct __common_reference_impl<_Tp1, _Tp2, 2>
> { using type = __basic_common_ref<_Tp1, _Tp2>; };
>
> // Otherwise, if COND-RES(T1, T2) is well-formed, ...
> template<typename _Tp1, typename _Tp2>
> - struct __common_reference_impl<_Tp1, _Tp2, 3,
> - void_t<__cond_res<_Tp1, _Tp2>>>
> + requires requires { typename __cond_res<_Tp1, _Tp2>; }
> + struct __common_reference_impl<_Tp1, _Tp2, 3>
> { using type = __cond_res<_Tp1, _Tp2>; };
>
> // Otherwise, if common_type_t<T1, T2> is well-formed, ...
> template<typename _Tp1, typename _Tp2>
> - struct __common_reference_impl<_Tp1, _Tp2, 4,
> - void_t<common_type_t<_Tp1, _Tp2>>>
> + requires requires { typename common_type_t<_Tp1, _Tp2>; }
> + struct __common_reference_impl<_Tp1, _Tp2, 4>
> { using type = common_type_t<_Tp1, _Tp2>; };
>
> // Otherwise, there shall be no member type.
> template<typename _Tp1, typename _Tp2>
> - struct __common_reference_impl<_Tp1, _Tp2, 5, void>
> + struct __common_reference_impl<_Tp1, _Tp2, 5>
> { };
>
> // Otherwise, if sizeof...(T) is greater than two, ...
> --
> 2.51.0.491.g4b71b29477
>
>
