append to variadic template

example.hpp

#include <template_append.hpp>
#include <tuple>

#define WITH_INT
#define WITH_DOUBLE

using type0 = std::tuple<>;
#ifdef WITH_INT
using type1 = JMAL::template_append<int, type0>::type;
#else
using type1 = JMAL::template_append<void, type0>::type;
#endif
#ifdef WITH_DOUBLE
using type2 = JMAL::template_append<double, type1>::type;
#else
using type2 = JMAL::template_append<void, type1>::type;
#endif

#if defined(WITH_INT) and defined(WITH_DOUBLE)
static_assert(std::is_same<type2,
                           std::tuple<int, double>>::value,
              "Didn't work");
#elif defined(WITH_INT)
static_assert(std::is_same<type2,
                           std::tuple<int>>::value,
              "Didn't work");
#elif defined(WITH_DOUBLE)
static_assert(std::is_same<type2,
                           std::tuple<double>>::value,
              "Didn't work");
#endif

template_append.hpp

#ifndef _JMAL_TEMPLATE_APPEND_H
#define _JMAL_TEMPLATE_APPEND_H

namespace JMAL
{

template <typename T, typename Seq>
struct template_append;

template <typename T, template<typename...> class Seq, typename... Args>
struct template_append<T, Seq<Args...>>
{
    using type = Seq<Args..., T>;
};

template <template<typename...> class Seq, typename... Args>
struct template_append<void, Seq<Args...>>
{
    using type = Seq<Args...>;
};

} // namespace JMAL

#endif