#include <type_traits>

template <typename...>
struct Pack {};

template <typename T, typename P>
struct Prepend;

template <typename T, typename... Ts>
struct Prepend<T, Pack<Ts...>>
{
    using type = Pack<T, Ts...>;
};

template <typename From, typename To, typename P>
struct Replace
{
    using type = P;
};

template <typename From, typename To, typename T, typename... Ts>
struct Replace<From, To, Pack<T, Ts...>>
{
    using type = typename Prepend<T, typename Replace<From, To, Pack<Ts...>>::type>::type;
};

template <typename From, typename To, typename... Ts>
struct Replace<From, To, Pack<From, Ts...>>
{
    using type = typename Prepend<To, typename Replace<From, To, Pack<Ts...>>::type>::type;
};

int main()
{
    // replace no doubles with void
    {
        using Input = Pack<int, float, char>;
        using Answer = Pack<int, float, char>;
        
        using Output = typename Replace<double, void, Input>::type;
        static_assert( std::is_same_v<Output, Answer> );
    }


    // Replace one float with double
    {
        using Input = Pack<int, float, char>;
        using Answer = Pack<int, double, char>;
        
        using Output = typename Replace<float, double, Input>::type;
        static_assert( std::is_same_v<Output, Answer> );
    }

    // Replace multiple floats with doubles
    {
        using Input = Pack<float, int, float, char>;
        using Answer = Pack<double, int, double, char>;
        
        using Output = typename Replace<float, double, Input>::type;
        static_assert( std::is_same_v<Output, Answer> );
    }

    // Replace multiple ints with float
    {
        using Input = Pack<float, int, float, char, int, int>;
        using Answer = Pack<float, float, float, char, float, float>;
        
        using Output = typename Replace<int, float, Input>::type;
        static_assert( std::is_same_v<Output, Answer> );
    }
}