[Bug libstdc++/110366] New: [C++26] P2641R4 Checking if a union alternative is active

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110366

            Bug ID: 110366
           Summary: [C++26] P2641R4 Checking if a union alternative is
                    active
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
            Blocks: 110339
  Target Milestone: ---

https://wg21.link/p2641r4

This needs a new built-in. We can't use __builtin_constant_p(&x) because the
address of a union member is a constant even if that isn't the active member.
We can't use __builtin_constant_p(x) because not all types usable in constexpr
contexts are boolean testable.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110339
[Bug 110339] Implement C++26 library features