Use of abstract tagged types without access types

["Prof. Dr. Carl Weierstrass" <carl.weierstrass@googlemail.com>]

Hi,

I have a question to all Ada gurus:

I want to do something like:

package Test is

   type Parent_Type is new Ada.Finalization.Controlled with private;
   type Child_Type is abstract new Ada.Finalization.Controlled with
private;
   type Child_Type_1 is new Child_Type with private;
   type Child_Type_N is new Child_Type with private;

private

   type Parent_Type is new Ada.Finalization.Controlled with record
         Child : Child_Type'Class;
   end record;
   type Child_Type is abstract new Ada.Finalization.Controlled with
null record;
   type Child_Type_1 is new Child_Type with null record;
   type Child_Type_N is new Child_Type with null record;

end Test;

This doesn't work, because:
class-wide subtype with unknown discriminants in component declaration

Of course I could