template<size_t N>
struct K_ { static constexpr size_t value = N; };

template <size_t N, size_t I, class Closure>
typename std::enable_if_t<(I == N)> is_loop(Closure& closure) {}

template <size_t N, size_t I, class Closure>
typename std::enable_if_t<(I < N)> is_loop(Closure& closure) {
   closure.template apply<I>();
   is_loop<N, I + 1>(closure);
}
template <size_t N, class Closure>
void loop_(Closure& closure) {
   is_loop<N, 1>(closure);
}
template <typename Closure>
struct is_sum_closure
{
   typedef typename Closure::value_type value_type;
   Closure& closure;
   value_type result;
   is_sum_closure(Closure& closure) : closure(closure), result(value_type()) {}

   template<size_t I>
   void apply() {
       result += closure.template value<I>();
   }
};
template <size_t N, class Closure>
typename Closure::value_type sum_(Closure& closure) {
   is_sum_closure<Closure> my_closure(closure);
   loop_<N>(my_closure);
   return my_closure.result;
}

struct sum_closure
{
   typedef double value_type;…