The non-static data member can share the address of
another non-static data member or that of a base class,
and any padding that would normally be inserted
at the end of the object
can be reused as storage for other members.
— end note]
Recommended practice: The value of a has-attribute-expression
for the no_unique_address attribute
should be 0 for a given implementation
unless this attribute can cause a potentially-overlapping subobject
to have zero size.