inner_join

#include <iostream>

template<class A, class B>
struct P;

template<class... Ts>
struct L {};

template<class T, class... Ts>
using mul = L<P<T, Ts>...>;

template<class...>
struct cat;

template<class T>
struct cat<T>
{
    using type = T;
};

template<class... As, class... Bs>
struct cat<L<As...>, L<Bs...>>
{
    using type = L<As..., Bs...>;
};

template<class A, class B, class... Ts>
struct cat<A, B, Ts...>
{
    using type = typename cat<typename cat<A, B>::type, Ts...>::type;
};

template<class A, class B>
struct join;

template<class... As, class... Bs>
struct join<L<As...>, L<Bs...>>
{
    using type = typename cat<mul<As, Bs...>...>::type;
};

template<class>
struct Print;

int main ()
{
    Print<join<L<int[1], int[2]>, L<float[1], float[2], float[3]>>::type>();
}