foo.cpp

#include <iostream>
#include <type_traits>
#include <typeinfo>
#include <typeindex>

template<typename T> struct Key;
template<> struct Key<int> { static constexpr std::size_t value = 0; };
template<> struct Key<float> { static constexpr std::size_t value = 1; };

template<typename T, typename U>
struct Sort
{
    using T1 = typename std::conditional<Key<T>::value <= Key<U>::value, T, U>::type;
    using T2 = typename std::conditional<Key<T>::value <= Key<U>::value, U, T>::type;
};

template<typename T, typename U>
struct A
{
    T t;
    U u;
};

template<typename T, typename U>
struct A_proxy
{
    A<typename Sort<T, U>::T1, typename Sort<T, U>::T2> s;
};

int main()
{
    A_proxy<int, float> a;
    a.s.t = 10;
    a.s.u = 0.1;
    std::cout << a.s.t << ' ' << a.s.u << std::endl;

    A_proxy<float, int> b;
    b.s.t = 10;
    b.s.u = 0.1;
    std::cout << b.s.t << ' ' << b.s.u << std::endl;

    a.s = b.s;                      // same!
}