[Bug libstdc++/111351] constexpr std::string objects permitted to escape constant evaluation when SSO

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111351

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Does using __builtin_is_constant_p on the union member not work?