P2781R9: `std::constant_wrapper`

[frederick-vs-ja]

Link: https://wg21.link/P2781R9

Feature-test macro

#define __cpp_lib_constant_wrapper 202506L // freestanding, also in <type_traits>

[Tsche]

Feel free to assign me to this issue. I've started preparing a patch :)

[ldionne]

@Tsche Do you have a PR ready to open? I'm wary of keeping something assigned to someone for too long if no PR is opened, since that can lead to stuff being dropped and nobody working on them because they think someone else is doing it.

[Tsche]

I'm not finished with the test suite yet and unfortunately discovered a wording defect. This has an issue number now (LWG4500), so work can continue.

Also GCC gave me a lot of trouble, it needs a lot of extra help to make sense of constant_wrapper as specified (libstdc++ currently implements an earlier spec). Not yet sure if this needs a new bug report for GCC, if it does I think we should refer to it.

Expect a PR soon :)

[ldionne]

I'm going to un-assign this issue for now since we don't want to keep things assigned without a PR for extended periods. This doesn't mean you can't work on this issue, just that we don't want to preclude others from putting up PRs.

[huixie90]

Discussed with @ldionne , I am going to create PR for this one as it is blocking the function_ref PRs #94894