std::common_reference_with
Defined in header <concepts>


template < class T, class U > concept common_reference_with = 
(since C++20)  
The concept common_reference_with<T, U>
specifies that two types T
and U
share a common reference type (as computed by std::common_reference_t) to which both can be converted.
[edit] Semantic requirements
T and U model common_reference_with<T, U>
only if, given equalitypreserving 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_reference_t<T, U>(t1) equals std::common_reference_t<T, U>(t2) if and only if
t1
equalst2
; and  std::common_reference_t<T, U>(u1) equals std::common_reference_t<T, U>(u2) if and only if
u1
equalsu2
.
In other words, the conversion to the common reference type must preserve equality.
[edit] Equality preservation
An expression is equality preserving if it results in equal outputs given equal inputs.
 The inputs to an expression consist of its operands.
 The outputs of an expression consist of its result and all operands modified by the expression (if any).
In specification of standard concepts, operands are defined as the largest subexpressions that include only:
 an idexpression, and
 invocations of std::move, std::forward, and std::declval.
The cvqualification and value category of each operand is determined by assuming that each template type parameter denotes a cvunqualified complete nonarray object type.
Every expression required to be equality preserving is further required to be stable: two evaluations of such an expression with the same input objects must have equal outputs absent any explicit intervening modification of those input objects.
[edit] See also
determines the common reference type of a group of types (class template)  
(C++20) 
specifies that two types share a common type (concept) 
(C++11) 
determines the common type of a group of types (class template) 