fix_pair_base_class.h

template <class T1, class T2, bool is_reference>
struct MyPairBase;

// Dispatch the reference specialisation and use it as a storage.
template <class T1, class T2>
struct MyPairBase<T1, T2, false> {
    T1 a;
    T2 b;
    MyPairBase& operator=(const MyPairBase& other) = default;
};

template <class T1, class T2>
struct MyPairBase<T1, T2, true> {
    T1 a;
    T2 b;
    MyPairBase& operator=(const MyPairBase& other) {
        a = other.a;
        b = other.b;
        return *this;
    }
    // Possibly we will need to copy many things there.
};

template <class T1, class T2>
struct MyPair : public MyPairBase<T1, T2, std::is_reference<T1>::value ||
                                          std::is_reference<T2>::value> {
    using Base = MyPairBase<T1, T2, std::is_reference<T1>::value ||
                                    std::is_reference<T2>::value>;
    // ... all the implementation
};