https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87389
--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> --- Specifically, the not-taken branch of an if constexpr is only discarded when inside a template. In a non-template both branches are compiled as normal, and so both static assertions get compiled. Since both cannot possibly be true at once, the program can never compile.
