enum variant static dispatcher

#include <type_traits>
#include <utility>

#include <cassert>

template< typename E, E first, E ...rest >
struct front
{

    static constexpr E value = first;

};

template< typename E, template< E > class I, E ...es >
struct D
{

    template< E e, typename ...Ts >
    static
    constexpr
    decltype(auto)
    call(Ts &&... ts)
    {
        return I< e >{}(std::forward< Ts >(ts)...);
    }

    template< typename ...Ts >
    using caller = decltype(&D::template call< front< E, es... >::value, Ts... >);

    template< typename ...Ts >
    static constexpr caller< Ts... > callers_[sizeof...(es)] = {D::template call< es, Ts... >...};

    template< std::size_t ...indices >
    constexpr
    std::size_t
    to_sequential_number(E e, std::index_sequence< indices... >) const
    {
        std::size_t i = 0;
        for (E ee : {es...}) {
            if (ee == e) {
                return i;
            }
            ++i;
        }
        assert(false);
    }

    template< typename ...Ts >
    constexpr
    decltype(auto) operator () (E e, Ts &&... ts) const
    {
        return callers_< Ts... >[to_sequential_number(e, std::make_index_sequence< sizeof...(es) >{})](std::forward< Ts >(ts)...);
    }

};

template< typename E, template< E > class I, E ...es >
template< typename ...Ts >
constexpr typename D< E, I, es... >::template caller< Ts... > D< E, I, es... >::callers_[sizeof...(es)];

#include <iostream>

#define PP { std::cout << __PRETTY_FUNCTION__ << std::endl; (std::cout << ... << ts) << std::endl; }

enum class E
{
    A,
    B,
    C,
};

template< E >
struct I;

template<>
struct I< E::A >
{

    template< typename ...Ts >
    decltype(auto) operator () (Ts &&... ts) PP

};

template<>
struct I< E::B >
{

    template< typename ...Ts >
    decltype(auto) operator () (Ts &&... ts) PP

};

template<>
struct I< E::C >
{

    template< typename ...Ts >
    decltype(auto) operator () (Ts &&... ts) PP

};

#include <cstdlib>

int
main()
{
    D< E, I, E::C, E::A, E::B >{}(E::A, 1, -2.1f, '3');
    return EXIT_SUCCESS;
}