cpp/concepts/common with

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

Semantic requirements
T and U model only if, given equality-preserving expressions, ,  and  such that  and  are both  and  and  are both ,
 * equals if and only if  equals ; and
 * equals if and only if  equals.

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