#include <type_traits>

struct X {};
struct Y {};
struct Z { Z(const X&); Z(const Y&);  };

X f(Z, Z);
template<class T, class U>
auto f(T t, U u) -> typename std::common_type<T, U>::type;

int main() {
  // std::common_type<X, Y>::type;
  X x;
  Y y;
  f(x, y);
}