https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70114
--- Comment #1 from joseph at codesourcery dot com <joseph at codesourcery dot com> --- On Mon, 7 Mar 2016, tanzx940228 at hotmail dot com wrote: > 3.gcc.error.c > ------------- > int main() { > { > int foo(float arg0, float arg1); > foo(3.0f, 4.0f); > } > { > foo(3.0f, 4.0f); // clang passes, gcc gets an error??? > } > } > > This is really weird! Clang gives me a warning saying that I'm using an > out-of-scope prototype. BUT gcc gives me an error, saying "incompatible > implicit declaration of function ‘foo’"! What happened? An unprototyped function type is never compatible with a function prototype with argument types subject to the default argument promotions, such as float. See C90 6.5.4.3 (reference to C90 since you're using implicit declarations).