How could unary operator & does not require a complete type?

The following code compiles fine with both gcc 7.2.0 and clang 6.0.0.

#include <iostream>

struct stru;

void func(stru& s) {
  std::cout << &s << std::endl;
}

int main() {

}

I'm wondering how this is OK. What if stru has overloaded operator&()? The compiler should not be able to tell with simply a forward declaration like struct stru. In my opinion, only std::addressof(s) is OK with an incomplete type.