[Bug libstdc++/104542] New: make_obj_using_allocator and uninitialized_construct_using_allocator lack constexpr

[de34 at live dot cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104542

            Bug ID: 104542
           Summary: make_obj_using_allocator and
                    uninitialized_construct_using_allocator lack constexpr
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: de34 at live dot cn
  Target Milestone: ---

std::make_obj_using_allocator and std::uninitialized_construct_using_allocator
are constexpr in C++20 (LWG3185, LWG3321), but currently they are not constexpr
in libstdc++.

I think the related compiler support for std::construct_at is available since
gcc11 (partially available in gcc10), so these functions are reasonably
constexpr in libstdc++ now, and this fix should be backported to gcc 10/11.

wandbox links:

gcc10 with manual constexpr (lacks support for CZ044):
https://wandbox.org/permlink/CvSEindXZa3MMRT5

gcc11&12 with manual constexpr:
https://wandbox.org/permlink/8i7oM16jGxlmz3FC
https://wandbox.org/permlink/VHQJhGINcdpNC0Eh

gcc11&12 with std versions, currently doesn't compile:
https://wandbox.org/permlink/ZUx1xZIOphwxcJhS
https://wandbox.org/permlink/f8uujdEoMUPbsUIq