https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88674
--- Comment #1 from joseph at codesourcery dot com <joseph at codesourcery dot com> --- "qualified" is used in the informal sense of "any additional specifiers along with void", not in the sense of "type qualifiers present". The program is not valid. J.2 explicitly lists "A storage-class specifier or type qualifier modifies the keyword void as a function parameter type list (6.7.6.3)." as undefined behavior. (I think this is a case of undefined-for-lack-of-semantics rather than normative-text-directly-says-is-undefined.)
