generic formal object of class-wide type

[Matthew Heaney <matthew_heaney@acm.org>]

When I try to compile package Q (see below), I get this error:

q.ads:3:25: type of actual does not match type of "O"

Why?  

Generic formal object O (in GQ) is of type P.T'Class.  

Actual object O is of specific type NT, yes, but that type is in
T'Class.  So why isn't object O allowed as the actual for formal GC.O?




package P is
   type T is abstract tagged limited null record;
end;


package P.C is
   type NT is new T with null record;
   O : NT;
end;


with P;
generic
  O : in out P.T'Class;
package GQ is
end;


with GQ, P.C;
package Q is new GQ (P.C.O);