std::common_with

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 common_with<T, U> specifies that two types T and U share a common type (as computed by std::common_type_t) to which both can be converted.

Semantic requirements

T and U model std::common_with<T,U> only if, given equality-preserving expressions t1, t2, u1 and u2 such that decltype((t1)) and decltype((t2)) are both T and decltype((u1)) and decltype((u2)) are both U,

• std::common_type_t<T, U>(t1) equals std::common_type_t<T, U>(t2) if and only if t1 equals t2; and
• std::common_type_t<T, U>(u1) equals std::common_type_t<T, U>(u2) if and only if u1 equals u2.

In other words, the conversion to the common type must preserve equality.

Equality preservation

Expressions declared in requires expressions of the standard library concepts are required to be equality-preserving (except where stated otherwise).

References

• C++23 standard (ISO/IEC 14882:2024):
• 18.4.6 Concept common_with [concept.common]
• C++20 standard (ISO/IEC 14882:2020):
• 18.4.6 Concept common_with [concept.common]

See also