::fibonacci_number

#include <type_traits>

namespace details {

  template<std::size_t Value>
  using constant = std::integral_constant<std::size_t, Value>;

  template<std::size_t>
  struct fibonacci_number;

  template<>
  struct fibonacci_number<0> : constant<0> { };

  template<>
  struct fibonacci_number<1> : constant<1> { };

  template<>
  struct fibonacci_number<2> : constant<1> { };

  template<std::size_t N>
  struct fibonacci_number {
    using _1 = typename fibonacci_number<N - 1>::type;
    using _2 = typename fibonacci_number<N - 2>::type;

    using type = constant<_1{} + _2{}>;
  };

}

template<std::size_t N>
struct fibonacci_number : details::fibonacci_number<N>::type { };