https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64509
--- Comment #3 from joseph at codesourcery dot com <joseph at codesourcery dot com> --- _Generic is intended for cases like <tgmath.h>, where calls to all functions are still valid for all argument types. This is a case where the expression __f(S) violates a constraint, and that constraint applies to all function calls, not just evaluated ones - this is just the same as a constraint violation inside if (0), or if (expression_that_evaluates_to_0_at_runtime). (It's also required that functions called in unevaluated generic associations are defined somewhere in the program; only sizeof and _Alignof have exemptions from that.)
