std::common_type(C++23)

namespace std {
  template<class T1, class T2, class U1, class U2>
    requires requires { typename pair<common_type_t<T1, U1>, common_type_t<T2, U2>>; }
  struct common_type<pair<T1, T2>, pair<U1, U2>> {
    using type = pair<common_type_t<T1, U1>, common_type_t<T2, U2>>;
  };
}