18 Concepts library [concepts]

18.4 Language-related concepts [concepts.lang]

18.4.6 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.
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C could be the same as T, or U, or it could be a different type.
C might not be unique.
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template<class T, class U> concept Common = Same<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>()); } && 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 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<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
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Users can customize the behavior of Common by specializing the common_­type class template ([meta.trans.other]).
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