This doesn't work in C because the function is not necessarily defined in the same translation unit.
You'd need to do this at link time instead, which would require completely overhauling the format of object files, dynamic libraries, static libraries so they carry information about the types of functions instead of just the symbol names. It's not an easy fix.
> This doesn't work in C because the function is not necessarily defined in the same translation unit.
It does work in C ― that's what the include files are for (among other things), after all. So it's possible to be able to forward-declare functions outside the translation unit only (extern-declare?), those inside the translation units don't need to even if the compiler works in a single pass. And those external declaration could still be introduced at the very end of the translation unit and still would count. I dunno, seems like a pretty reasonable idea.
That's what DWARF is for, and similar formats on other platforms. Also Pascal-lineage languages commonly use this kind of embedded type information to provide module interfaces, it works quite well.
You'd need to do this at link time instead, which would require completely overhauling the format of object files, dynamic libraries, static libraries so they carry information about the types of functions instead of just the symbol names. It's not an easy fix.