cpp/experimental/ranges/concepts/CommonReference

The concept specifies that two types  and  share a common reference type (as computed by ranges) to which both can be converted.

is satisfied only if, given expressions and  such that  is  and  is ,
 * is equal to if and only if  is an equality-preserving expression; and
 * is equal to if and only if  is an equality-preserving expression.

In other words, the conversion to the common reference type must not alter the equality-preservation property of the original expression.