J.11 The Class Attribute of Untagged Incomplete Types
Static Semantics
1/2
For the first subtype
S of a type T declared by an incomplete_type_declaration
that is not tagged, the following attribute is defined:
2/2
S'Class
Denotes the first subtype of the incomplete class-wide type rooted at
T. The completion of T shall declare a tagged type. Such
an attribute reference shall occur in the same library unit as the incomplete_type_declaration.