#include <iostream>
#include <concepts>

template<typename R, typename T, typename U>
requires std::equivalence_relation<R, T, U>
void f(const char* name, const char* tname, const char* uname) {
  std::cout << name << " is equivalence relation between " << tname << " and " << uname << std::endl;
}

template<typename R, typename T, typename U>
void f(const char* name, const char* tname, const char* uname) {
  std::cout << name << " is not equivalence relation between " << tname << " and " << uname << std::endl;
}


struct S1 {
  int n = 0;
};

struct S2 {
  short m = 0;
};

// S1とS2の間の同値関係
struct equivalence_relation_s1s2 {

  bool operator()(S1 lhs, S1 rhs) const {
    return lhs.n == rhs.n;
  }

  bool operator()(S2 lhs, S2 rhs ) const {
    return lhs.m == rhs.m;
  }

  bool operator()(S1 lhs, S2 rhs) const {
    return lhs.n == rhs.m;
  }

  bool operator()(S2 lhs, S1 rhs) const {
    return lhs.m == rhs.n;
  }
};

int main() {
  f<equivalence_relation_s1s2, S1, S2>("equivalence_relation_s1s2", "S1", "S2");
  f<equivalence_relation_s1s2, S2, S1>("equivalence_relation_s1s2", "S2", "S1");
}