https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111351
--- Comment #5 from James Y Knight <foom at fuhm dot net> --- > Does using __builtin_is_constant_p on the union member not work? I've created a proof-of-concept patch for libc++ to support SSO strings during constant evaluation. It works. If everyone disagrees with me and believes that this is a really awesome foot-gun to give to users, I will go ahead and propose that patch to libc++ maintainers. (As mentioned, that'll cause more code to be compilable under libc++ than is possible to permit under libstdc++/MSVC implementations). However, I continue to believe the opposite outcome, prohibiting this everywhere, would be preferable.
