On Mon, 13 Oct 2025, 09:01 Jonathan Wakely, <[email protected]> wrote:
> > > On Mon, 13 Oct 2025, 09:00 Jonathan Wakely, <[email protected]> wrote: > >> >> >> On Mon, 13 Oct 2025, 05:42 Patrick Palka, <[email protected]> wrote: >> >>> We implement this paper as a DR against C++20 (as do MSVC and libc++). >>> >>> PR libstdc++/120446 >>> >>> libstdc++-v3/ChangeLog: >>> >>> * include/bits/version.def (common_reference): New. >>> * include/bits/version.h: Regenerate. >>> * include/std/type_traits (__glibcxx_want_common_reference): >>> Define. >>> (__common_reference_impl<T1, T2, 1>): Add pointer convertibility >>> constraints as per P2655R3. >>> * testsuite/20_util/common_reference/p2655r3.cc: New test. >>> >> >> ok for trunk >> > > This looks safe for backports too, do we want to do that? > Ah you mentioned that in the patch 3/3 email. My vote would be yes, backport this part (which also requires backporting the first patch in the series). I'm undecided about the third part of the series. It's probably harmless to backport, but also not really critical to do. > > >> >> --- >>> libstdc++-v3/include/bits/version.def | 8 ++++++++ >>> libstdc++-v3/include/bits/version.h | 10 ++++++++++ >>> libstdc++-v3/include/std/type_traits | 7 +++++++ >>> .../testsuite/20_util/common_reference/p2655r3.cc | 15 +++++++++++++++ >>> 4 files changed, 40 insertions(+) >>> create mode 100644 >>> libstdc++-v3/testsuite/20_util/common_reference/p2655r3.cc >>> >>> diff --git a/libstdc++-v3/include/bits/version.def >>> b/libstdc++-v3/include/bits/version.def >>> index 83f1817bf8e4..e2b064e09b1e 100644 >>> --- a/libstdc++-v3/include/bits/version.def >>> +++ b/libstdc++-v3/include/bits/version.def >>> @@ -1793,6 +1793,14 @@ ftms = { >>> }; >>> }; >>> >>> +ftms = { >>> + name = common_reference; >>> + values = { >>> + v = 202302; >>> + cxxmin = 20; // We treat P2655R3 as a DR against C++20. >>> + }; >>> +}; >>> + >>> ftms = { >>> name = formatters; >>> values = { >>> diff --git a/libstdc++-v3/include/bits/version.h >>> b/libstdc++-v3/include/bits/version.h >>> index 0d6692d244a6..4581519d3a7d 100644 >>> --- a/libstdc++-v3/include/bits/version.h >>> +++ b/libstdc++-v3/include/bits/version.h >>> @@ -2001,6 +2001,16 @@ >>> #endif /* !defined(__cpp_lib_flat_set) */ >>> #undef __glibcxx_want_flat_set >>> >>> +#if !defined(__cpp_lib_common_reference) >>> +# if (__cplusplus >= 202002L) >>> +# define __glibcxx_common_reference 202302L >>> +# if defined(__glibcxx_want_all) || >>> defined(__glibcxx_want_common_reference) >>> +# define __cpp_lib_common_reference 202302L >>> +# endif >>> +# endif >>> +#endif /* !defined(__cpp_lib_common_reference) && >>> defined(__glibcxx_want_common_reference) */ >>> +#undef __glibcxx_want_common_reference >>> + >>> #if !defined(__cpp_lib_formatters) >>> # if (__cplusplus >= 202100L) && _GLIBCXX_HOSTED >>> # define __glibcxx_formatters 202302L >>> diff --git a/libstdc++-v3/include/std/type_traits >>> b/libstdc++-v3/include/std/type_traits >>> index 8b5110464e50..47cbf21b4c1d 100644 >>> --- a/libstdc++-v3/include/std/type_traits >>> +++ b/libstdc++-v3/include/std/type_traits >>> @@ -41,6 +41,7 @@ >>> >>> #define __glibcxx_want_bool_constant >>> #define __glibcxx_want_bounded_array_traits >>> +#define __glibcxx_want_common_reference >>> #define __glibcxx_want_constant_wrapper >>> #define __glibcxx_want_has_unique_object_representations >>> #define __glibcxx_want_integral_constant_callable >>> @@ -4223,6 +4224,12 @@ template<typename _Ret, typename _Fn, typename... >>> _Args> >>> template<typename _Tp1, typename _Tp2> >>> requires is_reference_v<_Tp1> && is_reference_v<_Tp2> >>> && requires { typename __common_ref<_Tp1, _Tp2>; } >>> +#if __cpp_lib_common_reference // C++ >= 20 >>> + && is_convertible_v<add_pointer_t<_Tp1>, >>> + add_pointer_t<__common_ref<_Tp1, _Tp2>>> >>> + && is_convertible_v<add_pointer_t<_Tp2>, >>> + add_pointer_t<__common_ref<_Tp1, _Tp2>>> >>> +#endif >>> struct __common_reference_impl<_Tp1, _Tp2, 1> >>> { using type = __common_ref<_Tp1, _Tp2>; }; >>> >>> diff --git a/libstdc++-v3/testsuite/20_util/common_reference/p2655r3.cc >>> b/libstdc++-v3/testsuite/20_util/common_reference/p2655r3.cc >>> new file mode 100644 >>> index 000000000000..4188dd2fb29f >>> --- /dev/null >>> +++ b/libstdc++-v3/testsuite/20_util/common_reference/p2655r3.cc >>> @@ -0,0 +1,15 @@ >>> +// P2655R3 - common_reference_t of reference_wrapper Should Be a >>> Reference Type >>> +// Implemented as a DR against C++20 >>> +// { dg-do compile { target c++20 } } >>> + >>> +#include <type_traits> >>> + >>> +#if __cpp_lib_common_reference != 202302L >>> +# error "Feature-test macro __cpp_lib_common_reference has wrong value >>> in <type_traits>" >>> +#endif >>> + >>> +struct A { }; >>> +struct B { operator A&() const; }; >>> + >>> +static_assert( std::is_same_v<std::common_reference_t<A&, const B&>, >>> A&> ); >>> +static_assert( std::is_same_v<std::common_reference_t<const B&, A&>, >>> A&> ); >>> -- >>> 2.51.0.491.g4b71b29477 >>> >>>
