Making sense of predicates

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

Considering this:

   type T is tagged null record;
   procedure P (X : T);

   type S is new T with null record;
   procedure Q (X : T);

   subtype Subclass is T'Class with
      Dynamic_Predicate => Subclass in S'Class;
      -- Should have been Static_Predicate, but illegal

Now:

   X : Subclass := S'(null record);
begin
   X.P;
   X.Q; -- Illegal

Well, predicates were not supposed to work in an intelligent way, so why
not to add a kind that will?

There are number of cases where this would be very useful. E.g. smart
pointers:

   type T1 is tagged ...;
   type T1_Ptr is access all T1'Class;
   type H1 is new Ada.Finalization.Controlled with record -- Handle type
       Ptr : T1_Ptr;
   end record;

   type T2 is new T1 with ...;

Presently it is impossible to derive H2 from H1 constrained to point to
T2'Class. (Neither it were possible with implicit dereferencing)

With the above, one would create a subtype of H1 constrained to T2'Class
and then derive from it (better in one step, of course).

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