On Tue, 9 Sept 2025 at 13:15, Tomasz Kaminski <tkami...@redhat.com> wrote:
>
>
>
> On Tue, Sep 9, 2025 at 12:45 PM Jonathan Wakely <jwak...@redhat.com> wrote:
>>
>> On Fri, 5 Sept 2025 at 08:14, Tomasz Kamiński <tkami...@redhat.com> wrote:
>> >
>> > From: Luc Grosheintz <luc.groshei...@gmail.com>
>> >
>> > This is a partial implementation of P2781R9. It adds std::cw and
>> > std::constant_wrapper, but doesn't modify __integral_constant_like for
>> > span/mdspan.
>> >
>> > libstdc++-v3/ChangeLog:
>> >
>> > * include/bits/version.def (constant_wrapper): Add.
>> > * include/bits/version.h: Regenerate.
>> > * include/std/type_traits (_CwFixedValue): New class.
>> > (_IndexSequence): New struct.
>> > (_BuildIndexSequence): New struct.
>> > (_ConstExprParam): New concept.
>> > (_CwOperators): New struct.
>> > (constant_wrapper): New struct.
>> > (cw): New global constant.
>> > * src/c++23/std.cc.in (constant_wrapper): Add.
>> > (cw): Add.
>> > * testsuite/20_util/constant_wrapper/adl.cc: New test.
>> > * testsuite/20_util/constant_wrapper/ex.cc: New test.
>> > * testsuite/20_util/constant_wrapper/generic.cc: New test.
>> > * testsuite/20_util/constant_wrapper/instantiate.cc: New test.
>> > * testsuite/20_util/constant_wrapper/op_comma_neg.cc: New test.
>> > * testsuite/20_util/constant_wrapper/version.cc: New test.
>> >
>> > Co-authored-by: Tomasz Kamiński <tkami...@redhat.com>
>> > Signed-off-by: Luc Grosheintz <luc.groshei...@gmail.com>
>> > Signed-off-by: Tomasz Kamiński <tkami...@redhat.com>
>> > ---
>> > v5 adds data member pointer tests
>>
>> Some late comments that I should have noticed before approving it, sorry!
>>
>>
>> > diff --git a/libstdc++-v3/include/std/type_traits
>> > b/libstdc++-v3/include/std/type_traits
>> > index 4636457eb5a..26cbbb4fd5b 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_constant_wrapper
>> > #define __glibcxx_want_has_unique_object_representations
>> > #define __glibcxx_want_integral_constant_callable
>> > #define __glibcxx_want_is_aggregate
>> > @@ -4302,6 +4303,376 @@ template<typename _Ret, typename _Fn, typename...
>> > _Args>
>> > };
>> > #endif // C++11
>> >
>> > +#ifdef __cpp_lib_constant_wrapper // C++ >= 26
>> > + template<typename _Tp>
>> > + struct _CwFixedValue
>> > + {
>> > + using _S_type = _Tp;
>>
>> The "_S_" prefix is for static members, this should be just "__type".
>> I'll change this.
>>
>>
>> > +
>> > + template<_CwFixedValue _Tp,
>> > + typename = typename decltype(_CwFixedValue(_Tp))::_S_type>
>> > + struct constant_wrapper;
>>
>> Why do we need decltype(_CwFixedValue(_Tp)) here?
>>
>> It looks to me like just decltype(_Tp)::_S_type should work ... but it
>> doesn't. Is that something to do with CTAD being involved?
>
> I think this is a GCC bug, that got a workaround backed in the standard
> (the standard specifies the argument this way, this is why it haven't
> picked up in review). Just decltype(_Tp) works in clang.
I've submitted https://github.com/cplusplus/draft/pull/8218 to
simplify the C++26 draft.
