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From: Adam Beneschan <adam@irvine.com>
Newsgroups: comp.lang.ada
Subject: Re: Extending discriminant types
Date: Mon, 17 Nov 2008 08:30:45 -0800 (PST)
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Date: 2008-11-17T08:30:45-08:00
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On Nov 15, 1:16 am, Stefan Bellon <bel...@software-erosion.org> wrote:
> 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?

No, I think GNAT is wrong here.  The semantics of allocators is that
an object is created with K=A, and then it tries to convert this
object to the subtype Child, but since Child has a constraint and the
object violates that constraint, a Constraint_Error should be raised.
However, I think this would be OK and would not raise an exception:

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

since there are no constraints to violate.  I'm not 100% sure I'm
right about this, though.

                                 -- Adam