18 Concepts library [concepts]

18.4 Language-related concepts [concepts.lang]

18.4.6 Concept common_with [concept.common]

If T and U can both be explicitly converted to some third type, C, then T and U share a common type, C.
[Note 1: 
C can be the same as T or U, or can be a different type.
C is not necessarily unique.
— end note]
template<class T, class U> concept common_with = same_as<common_type_t<T, U>, common_type_t<U, T>> && requires { static_cast<common_type_t<T, U>>(declval<T>()); static_cast<common_type_t<T, U>>(declval<U>()); } && common_reference_with< add_lvalue_reference_t<const T>, add_lvalue_reference_t<const U>> && common_reference_with< add_lvalue_reference_t<common_type_t<T, U>>, common_reference_t< add_lvalue_reference_t<const T>, add_lvalue_reference_t<const U>>>;
Let C be common_type_t<T, U>.
Let t1 and t2 be equality-preserving expressions ([concepts.equality]) such that decltype((t1)) and decltype((t2)) are each T, and let u1 and u2 be equality-preserving expressions such that decltype((u1)) and decltype((u2)) are each U.
T and U model common_with<T, U> only if:
  • C(t1) equals C(t2) if and only if t1 equals t2, and
  • C(u1) equals C(u2) if and only if u1 equals u2.
[Note 2: 
Users can customize the behavior of common_with by specializing the common_type class template ([meta.trans.other]).
— end note]