https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114816
--- Comment #1 from Joseph S. Myers <jsm28 at gcc dot gnu.org> ---
Note that it's not possible to define a function with such parameters ("After
adjustment, the parameters in a parameter type list in a function declarator
that is part of a definition of that function shall not have incomplete
type.").
"register void" and "const void" (single unnamed parameter) are in the list of
undefined behavior in Annex J ("A storage-class specifier or type qualifier
modifies the keyword void as a function parameter type list") - presumably
implicitly undefined for lack of definition. As for named void parameters (or
more than one void parameter, etc.), the response to C99 DR#295 says "The
Committee agrees that there do not appear to be any constraints forbidding
constructions like void func(void parm); nor are any semantics provided for
this construction." (which would also be implicitly undefined behavior).
Given the desire to reduce undefined behavior in C2y it would be reasonable to
submit a paper tightening this up by making all the problem cases (any case of
a void parameter except for the special case of a single unnamed parameter of
type void, no storage class specifier, no ellipsis following) into constraint
violations for function declarators - or indeed to raise this through the issue
tracking process once we have the new issue tracker set up, with a view to
inviting WG14 to consider this a defect applicable to older standard versions.
