std::flat_multimap::key_equiv(C++23)

struct key_equiv {
  key_equiv(key_compare c) : comp(c) { } // (1) C++23
constexpr
  key_equiv(key_compare c) : comp(c) { } // (1) C++26

  bool
    operator()(const_reference x, const_reference y) const { // (2) C++23
      return !comp(x.first, y.first) && !comp(y.first, x.first);
    }
  constexpr bool
    operator()(const_reference x, const_reference y) const { // (2) C++26
      return !comp(x.first, y.first) && !comp(y.first, x.first);
    }

  key_compare comp;
};