C++ is fun until your compiler shoots into your leg
struct A {
using T = T1;
using U = U1;
operator U1 T1::*();
operator U1 T2::*();
operator U2 T1::*();
operator U2 T2::*();
};
inline auto which(U1 T1::*) { return "gcc"; }
inline auto which(U1 T2::*) { return "icc"; }
inline auto which(U2 T1::*) { return "msvc"; }
inline auto which(U2 T2::*) { return "clang"; }
int main() {
A a;
using T = T2;
using U = U2;
puts(which(a.operator U T::*()));
}