`conversion_dispatch` (aka. `explicit_conversion_dispatch`) won't work for reference-retuning overloads

[mingxwa]

The follow code is well-formed but will be rejected:

#include <proxy/proxy.h>

struct Base {};
struct Derived : Base {};

struct MyFacade : pro::facade_builder
    ::add_convention<pro::conversion_dispatch, Base&()>
    ::build {};

int main() {
  static_assert(pro::proxiable<Derived*, MyFacade>);
}