I regularly use that in C, to make sure a function matches an abstract interface. Sure, that often ends up in a function pointer, but not always and when I declare the type signature, it isn't yet a function pointer.

> but not define

I think that is because the type signature only contains the types, but no parameter names, which are required for a definition. This is arbitrary, since for data types, the member names are part of the type. It sounds totally fixable, but then you either have two types of function types, one where all parameter names are qualified and one where they aren't and only could use the former for function definitions. Or you would make names also mandatory for function declarations.

> It sounds totally fixable, but then you either have two types of function types, one where all parameter names are qualified and one where they aren't and only could use the former for function definitions

Making the names part of the type would be a bit weird, although we have seen stranger things. The biggest problem is that it would be a breaking change at least in C++.

Exactly, I believe that to be the case in C as well. In C23 the rules for types to be considered compatible were actually relaxed, so this proposal wouldn't make any sense. It would be a useless breaking change for no gain, other than a bit less typing and maybe feeding some language layers, so there is really no reason to do that. It actually makes the code less clear, since you now need to always lookup the type definition, which would be against C's philosophy.