[Bug c++/125821] Implement C++29 language features

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125821

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Patch to add -std=c++29:
<https://gcc.gnu.org/pipermail/gcc-patches/2026-June/720164.html>