type_level_addition.cpp

#include<type_traits>
#include<iostream>

struct nat {};

template<typename T>
using is_nat = std::enable_if_t<std::is_base_of<nat, T>::value>;

template<typename N, typename = is_nat<N>>
struct suc : nat {};

struct _0 : nat {};

using _1 = suc<_0>;
using _2 = suc<_1>;
using _3 = suc<_2>;
using _4 = suc<_3>;
using _5 = suc<_4>;
using _6 = suc<_5>;
using _7 = suc<_6>;
using _8 = suc<_7>;
using _9 = suc<_8>;
using _10 = suc<_9>;

template<typename N, typename M, typename = is_nat<N>, typename = is_nat<M>>
struct add {};

template<typename N>
struct add<N, _0> { using type = N; };

template<typename N, typename M>
struct add<N, suc<M>> { using type = typename add<suc<N>, M>::type; };

template<typename N, typename = is_nat<N>>
struct to_int {};

template<>
struct to_int<_0> { constexpr static int value = 0; };

template<typename N>
struct to_int<suc<N>> { constexpr static int value = 1 + to_int<N>::value; };

int main() {
    std::cout << to_int<add<_3, _7>::type>::value << std::endl;
    std::cout << to_int<add<_5, _10>::type>::value << std::endl;
}