17 Concepts library [concepts]

17.4 Language-related concepts [concepts.lang]

17.4.6 Concept Common [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

C could be the same as T, or U, or it could be a different type.

C might not be unique.

— end note

]

🔗

template<class T, class U>
  concept Common =
    Same<common_type_t<T, U>, common_type_t<U, T>> &&
    ConvertibleTo<T, common_type_t<T, U>> &&
    ConvertibleTo<U, common_type_t<T, U>> &&
    CommonReference<
      add_lvalue_reference_t<const T>,
      add_lvalue_reference_t<const U>> &&
    CommonReference<
      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 t be a function whose return type is T, and let u be a function whose return type is U.

Common<T, U> is satisfied only if:

(2.1)
C(t()) equals C(t()) if and only if t() is an equality-preserving expression ([concepts.equality]).
(2.2)
C(u()) equals C(u()) if and only if u() is an equality-preserving expression ([concepts.equality]).

[ Note

Users can customize the behavior of Common by specializing the common_type class template ([meta.trans.other]).

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]