I wonder whether it makes sense and whether it would work to write the
following

extern "C" {
 template<typename R, typename ...P>
 using extern_C = R(P...);
}

So that you could for example write

void f(extern_C<void, int> *fp);

And that this would be equivalent to

extern "C" void extern_C(int);
void f(extern_C *fp);

That would allow to create extern-C function types on the fly, useful for
casting or such.

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