https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98945
--- Comment #3 from joseph at codesourcery dot com <joseph at codesourcery dot com> --- The relevant rule in ISO C is 6.7.6.3#15 (in C17): "For two function types to be compatible ... If one type has a parameter type list and the other type is specified by a function declarator that is not part of a function definition and that contains an empty identifier list, ... the type of each parameter shall be compatible with the type that results from the application of the default argument promotions". Thus, a declaration with a float argument is incompatible with an unprototyped declaration. *However* I see that when I implemented N2432 (removal of old-style function definitions) for C2x mode, I missed that that paper also removed that rule (keeping only the rule that an unprototyped declaration is incompatible with a variadic prototype). It's not obvious to me that such a change was intentional and it doesn't appear to have been discussed (it wasn't the main point of that paper), and it's still the case that you can't actually call a function with (float) prototype if only an unprototyped declaration is in scope, without undefined behavior; I'll raise that question on the WG14 reflector.
