concepts_pair.h

#include <type_traits>

template<class T1, class T2>
struct MyPair {
    T1 first;
    T2 second;
    static constexpr bool has_references = std::is_reference_v<T1> ||
                                           std::is_reference_v<T2>;
    
    MyPair(const T1& x, const T2& y) : first(x), second(y) {}

    MyPair& operator=(const MyPair&) requires(!has_references) = default;
    
    MyPair& operator=(const MyPair& other) requires(has_references) {
        first = other.first;
        second = other.second;
        return *this;
    }
};

int main() {
    int x = 10;
    MyPair<int&, int> a(x, 5);
    MyPair<int&, int> b(x, 10);
    b = a;
}