std:: common_with

Defined in header <concepts>
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 >>> ;		(since C++20)

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

See also

common_type (C++11)	determines the common type of a group of types (class template)
common_reference basic_common_reference (C++20)	determines the common reference type of a group of types (class template)
common_reference_with (C++20)	specifies that two types share a common reference type (concept)