https://gcc.gnu.org/g:95f5abd112020f86aba51ed35a6d517fb09682fe
commit r15-10609-g95f5abd112020f86aba51ed35a6d517fb09682fe Author: Patrick Palka <[email protected]> Date: Fri Dec 5 13:43:26 2025 -0500 libstdc++: Consolidate bullet 1 __common_reference_impl partial specs ... 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. Reviewed-by: Tomasz Kamiński <[email protected]> Reviewed-by: Jonathan Wakely <[email protected]> (cherry picked from commit cbdbbdd1fccfd789e6fbcb37b1b602bb7482de4b) Diff: --- libstdc++-v3/include/std/type_traits | 40 ++++++++++++------------------------ 1 file changed, 13 insertions(+), 27 deletions(-) diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits index 676cdf2d7e66..0a402617c52d 100644 --- a/libstdc++-v3/include/std/type_traits +++ b/libstdc++-v3/include/std/type_traits @@ -4113,7 +4113,7 @@ 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> { }; @@ -4126,46 +4126,32 @@ 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, ... @@ -4184,7 +4170,7 @@ template<typename _Ret, typename _Fn, typename... _Args> { }; /// @endcond -#endif // C++2a +#endif // C++20 /// @} group metaprogramming
