[Bug 293364] base libc++ incompatibilities with GCC 15, built-in traits

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=293364

            Bug ID: 293364
           Summary: base libc++ incompatibilities with GCC 15, built-in
                    traits
           Product: Base System
           Version: CURRENT
          Hardware: Any
               URL: https://github.com/llvm/llvm-project/pull/137871
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: bin
          Assignee: bugs@FreeBSD.org
          Reporter: mandree@FreeBSD.org
                CC: salvadore@freebsd.org, secteam@FreeBSD.org
            Blocks: 293329
             Flags: mfc-stable15?, mfc-stable14?, mfc-stable13?,
                    needs_errata?(secteam@FreeBSD.org)
                CC: secteam@FreeBSD.org

Greetings,

GCC 15 bombs out when building C++ stuff whilst using libc++ as its -stdlib=,
which is required to obtain ABI compatibility with requisite libraries built
through other ports.

However, libc++'s headers apparently use built-ins such as __decay with GCC 15
in places where GCC 14 and older would not, and GCC complains.

A good explanation is this comment:
https://github.com/NVIDIA/stdexec/issues/1375#issuecomment-2303803306

The Upstream LLVM project seems to have fixed this through
https://github.com/llvm/llvm-project/pull/137871 - can we cherry-pick these for
FreeBSD 15, 14, 13's libc++?

If not, what else would be the recommended solution? Is there some way to
override this?

There are related issues with suggestions that haven't materialized.
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=260139#c3
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=260139#c4

Unsure what this would give us:
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=215193


Referenced Bugs:

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=293329
[Bug 293329] graphics/rawtherapee: Fails to build with GCC 15: fatal error:
bits/functexcept.h: No such file or directory
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