std::basic_common_reference<std::pair> - cppreference.com

template< class T1, class T2, class U1, class U2,
          template<class> class TQual, template<class> class UQual >
  requires requires { typename std::pair<std::common_reference_t<TQual<T1>, UQual<U1>>,
                                         std::common_reference_t<TQual<T2>, UQual<U2>>>; }
struct basic_common_reference<std::pair<T1, T2>, std::pair<U1, U2>, TQual, UQual>;