Specialization generic package parameters

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

Recently I ran into the following problem with generic packages. Consider

   type T is tagged null record;

and a generic package that uses it:

   generic
      type Some_T is new T with private;
   package P is
      ...
   private
      ...
   end P;

Let there be some type S derived from T:

   type S is new T with null record;

Now the question is, how to derive another package Q from P, specialized
for S, while keeping access to the private part of P?

Neither of patterns I know works.

1. Nested package:

   generic
      type Some_S is new S with private;
   package Q is
      package P_S is new P (Some_S); -- This specializes P
   private
      -- No access to the private part of P!
   end Q;

2. Formal package:

   generic
      type Some_S is new S with private;
      package P_S is new P (Some_S); -- This specializes P
   package Q is
      ...
   private
      -- No access to the private part of P!
   end Q;

3. Child package

   generic
   package P.Q is
   private
       -- Got access to the private part, but
       -- lost specialization, as the parameter
       -- of P is no more constrained to S.
   end P;

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de