fibonacci in tail recursion template

fibo.cc

#include <iostream>

template<int N>
struct fibo_result { int val = N; };

template<int ...Ns>
struct fibo_impl;

template<int a, int b, int I, int ...Ns>
struct fibo_impl<a, b, I, I, Ns...>
{
  using type = fibo_result<a>;
};

template<int a, int b, int I, int ...Ns>
struct fibo_impl<a, b, I, Ns...>
{
  using type = typename fibo_impl<b, a+b, I+1, Ns...>::type;
};

template<int N>
int fibo()
{
  return typename fibo_impl<0, 1, 0, N>::type{}.val;
}

int main()
{
  std::cout << "fibo(45): " << fibo<45>() << std::endl;
}