Code like:
#include <signal.h>
int main() { _Generic(signal, int: 0); }
should fail with
error: type 'extern void (*(int, void (*)(int)))(int)' does not match any association
not
error: type 'extern void *(int)(int, void *(int))' does not match any association
[matz: fix formatting, fix function-to-pointer decay for operands of
_Generic, add testcase for this]