On Mon, 13 Oct 2025, 08:26 Tomasz Kaminski, <[email protected]> wrote:
> > > 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. > ... including the "C++2a" comment tweak, but nevermind, still ok for trunk. 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 >> >>
