View source for cpp/concepts/common with

{{cpp/title|common_with {{mark since c++20}}}}
{{cpp/concepts/navbar}}
{{ddcl|header=concepts|since=c++20|1=
template< class T, class U >
concept common_with =
    std::same_as<std::common_type_t<T, U>, std::common_type_t<U, T>> &&
    requires {
        static_cast<std::common_type_t<T, U>>(std::declval<T>());
        static_cast<std::common_type_t<T, U>>(std::declval<U>());
    } &&
    std::common_reference_with<
        std::add_lvalue_reference_t<const T>,
        std::add_lvalue_reference_t<const U>> &&
    std::common_reference_with<
        std::add_lvalue_reference_t<std::common_type_t<T, U>>,
        std::common_reference_t<
            std::add_lvalue_reference_t<const T>,
            std::add_lvalue_reference_t<const U>>>;
}}
The concept {{tt|common_with<T, U>}} specifies that two types {{tt|T}} and {{tt|U}} share a ''common type'' (as computed by {{lc|std::common_type_t}}) to which both can be converted.

===Semantic requirements===
T and U model {{co|std::common_with<T, U>}} only if, given [[cpp/concepts#Equality preservation|equality-preserving]] expressions {{tt|t1}}, {{tt|t2}}, {{tt|u1}} and {{tt|u2}} such that {{c|decltype((t1))}} and {{c|decltype((t2))}} are both {{tt|T}} and {{c|decltype((u1))}} and {{c|decltype((u2))}} are both {{tt|U}},
* {{c|std::common_type_t<T, U>(t1)}} equals {{c|std::common_type_t<T, U>(t2)}} if and only if {{tt|t1}} equals {{tt|t2}}; and
* {{c|std::common_type_t<T, U>(u1)}} equals {{c|std::common_type_t<T, U>(u2)}} if and only if {{tt|u1}} equals {{tt|u2}}.

In other words, the conversion to the common type must [[cpp/concepts#Equality preservation|preserve equality]].

{{cpp/concepts/equality preservation|skip-requires=yes}}

===References===
{{ref std c++23}}
{{ref std|section=18.4.6|title=Concept {{tt|common_with}}|id=concept.common}}
{{ref std end}}
{{ref std c++20}}
{{ref std|section=18.4.6|title=Concept {{tt|common_with}}|id=concept.common}}
{{ref std end}}

===See also===
{{dsc begin}}
{{dsc inc|cpp/types/dsc common_type}}
{{dsc inc|cpp/types/dsc common_reference}}
{{dsc inc|cpp/concepts/dsc common_reference_with}}
{{dsc end}}

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