Extending discriminant types

[Stefan Bellon <bellon@software-erosion.org>]

Hi,

I stumbled across some behaviour when extending an unconstrained
discriminant type while providing the constraint when extending. Assume
the following code:

   type Kind is (A, B);

   type Base (K : Kind) is abstract tagged record
      case K is
         when A => null;
         when B => Dummy : Natural;
      end case;
   end record;

   type Base_Access is access all Base'Class;

   type Child is new Base (B) with null record;

In this case I would have thought that type Child is now constrained to
Kind B and no object of type Child with Kind A can be created.

However, the following compiles (with GNAT) and does not throw an
exception at runtime:

   Var : Base_Access := new Child'(K => A);

Am I misunderstanding the Ada semantics? If so, is there a way to
achieve what I'm trying, i.e. that I can determine the Kind while
extending and not change it when creating objects?

Greetings,
Stefan