https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94032
--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> --- We'll add it when it's in the draft standard. Adding it before then would either mean only making it available for C++20 mode, or adding it to the shared library exports as a stable ABI artefact. Neither seems very good to me.
