[Note 4 : A complete-class context of a nested class is also a complete-class context of any enclosing class, if the nested
class is defined within the member-specification of the enclosing class. — end note]
7 A class is considered a completely-defined object type (6.8) (or complete type) at the closing } of the
class-specifier. The class is regarded as complete within its complete-class contexts; otherwise it is regarded
as incomplete within its own class member-specification.