https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66618
--- Comment #2 from joseph at codesourcery dot com <joseph at codesourcery dot
com> ---
Although diagnosing this probably makes sense, it's not required by the
standard ("An implementation may accept other forms of constant
expressions." - and this expression doesn't contain "assignment,
increment, decrement, function-call, or comma operators", so isn't
required by the Constraints for constant expressions not to be one).
