This "shall" is not listed under "Constraints", so violation of it is undefined behavior and does not require a diagnostic. See 4 (Conformance): If a ‘‘shall’’ or ‘‘shall not’’ requirement that appears outside of a constraint is violated, the behavior is undefined.

This is an area where a compiler can (sometimes) see violations, though, so I think gcc should diagnose when possible, in the same vein as format specifier mismatches in printf().