cpp/language/attributes/no unique address

Allows this data member to be overlapped with other non-static data members or base class subobjects of its class.

Explanation
Applies to the name being declared in the declaration of a non-static data member that's not a bit-field.

Makes this member subobject potentially-overlapping, i.e., allows this member to be overlapped with other non-static data members or base class subobjects of its class. This means that if the member has an empty class type (e.g. stateless allocator), the compiler may optimise it to occupy no space, just like if it were an empty base. If the member is not empty, any tail padding in it may be also reused to store other data members.