a.cc

template<int n>
struct IntT {};

template<typename T, int n>
T& operator<<(T& t, IntT<n>) {
  return t << n;
}

template<int n, int m>
constexpr bool operator==(IntT<n>, IntT<m>) {
  return n == m;
}

template<int n>
constexpr bool operator==(IntT<n>, int m) {
  return n == m;
}

template<int m>
constexpr bool operator==(int n, IntT<m>) {
  return n == m;
}

template<int n, int m>
constexpr IntT<n + m> operator+(IntT<n>, IntT<m>) {
  return {};
}

template<int n>
constexpr int operator+(IntT<n>, int m) {
  return n + m;
}

template<int m>
constexpr int operator+(int n, IntT<m>) {
  return n + m;
}

template<int n, int m>
constexpr IntT<n - m> operator-(IntT<n>, IntT<m>) {
  return {};
}

template<int n>
constexpr int operator-(IntT<n>, int m) {
  return n - m;
}

template<int m>
constexpr int operator-(int n, IntT<m>) {
  return n - m;
}

template<int n>
constexpr IntT<n> Int = {};

template<typename T>
constexpr auto Sum(T n) {
  if constexpr (n == Int<0>) {
    return n;
  } else {
    return Sum(n-Int<1>) + n;
  }
}

constexpr auto Sum(int n) {
  if (n == Int<0>) {
    return n;
  } else {
    return Sum(n-Int<1>) + n;
  }
}

#include <iostream>

int main() {
  std::cout << Sum(Int<3>) << "\n";
  int n = 3;
  std::cout << Sum(n) << "\n";
}