Note that some CFI (control flow integrity) implementations will get upset if you call a function pointer with the wrong argument types:

https://gcc.godbolt.org/z/EaPqKfvne

You could get around this by using a wrapper function, at the cost of a slightly different interface:

    template <typename T, void (*fn)(T *)>
    void wrapper(void *d) {
        fn((T *)d);
    }

    template <typename T, void (*fn)(T *)>
    Func0 MkFunc0(T* d) {
        auto res = Func0{};
        res.fn = (void *)wrapper<T, fn>;
        res.userData = (void*)d;
        return res;
    }

    ...

    Func0 x = MkFunc0<int, my_func>(nullptr);

(This approach also requires explicitly writing the argument type. It's possible to remove the need for this, but not without the kind of complexity you're trying to avoid.)