How to use "using" keyword for variadic template

I have a variadic template class that can take any number of variables in the constructor and also a std::tuple/std::pair and so forth.

I would like to use this wrapper for functions with varying return types.

For example:

class f1
{
    using output = double;
    output operator() { do_smth };
}
class f2
{
    using output = std::tuple<double,int>;
    output operator() { do_smth };
}



template <typename... Types>
class OutputType
{
    std::tuple<Types...> m_val;
public:
    OutputType(std::tuple<Types...>&& val) : m_val(val) {};
    OutputType(Types&& ... val) : m_val(std::forward<Types>(Types)...) {};
};

Now in a third class I would like to declare using like this:

template <typename F>
class dummy
{
    using Output = typename OutputType(typename F::Output));
}

How do I declare the above using statement so it does the correct thing also for dummy<f2>?

(i.e OutputType<double,int> and not OutputType<std::tuple<double,int>>)