[PATCH v2 2/3] libstdc++: Implement P2655R3 changes to common_reference bullet 1

[Tomasz Kaminski]

On Mon, Oct 13, 2025 at 10:11 AM Jonathan Wakely <jwakely.gcc@gmail.com>
wrote:

>
>
> On Mon, 13 Oct 2025, 09:01 Jonathan Wakely, <jwakely.gcc@gmail.com> wrote:
>
>>
>>
>> On Mon, 13 Oct 2025, 09:00 Jonathan Wakely, <jwakely.gcc@gmail.com>
>> wrote:
>>
>>>
>>>
>>> On Mon, 13 Oct 2025, 05:42 Patrick Palka, <ppalka@redhat.com> 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.
>
I would vote for backporting the first 2 patches (they are fixes), but not
third.

>
>
>>
>>
>>>
>>> ---
>>>>  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
>>>>
>>>>
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