pair_assignment.h

template <typename T, typename U>
struct pair {
    T first;
    U second;
};

int main() {
    int x = 10;
    int y = 20;
    pair<int&, int> p{x, x};
    pair<int&, int> p1{y, y};
    p1 = p; // ill-formed, with std::pair, well-formed
}