Ada 2012 Reference Manual

J.11 The Class Attribute of Untagged Incomplete Types

Static Semantics

For the first subtype S of a type T declared by an incomplete_type_declaration that is not tagged, the following attribute is defined:

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.