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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Interesting effects in array renaming
Date: Tue, 24 Jun 2003 14:48:46 +0200
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Date: 2003-06-24T14:48:46+02:00
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Georg Bauhaus wrote:

> Dmitry A. Kazakov <mailbox@dmitry-kazakov.de> wrote:
> : That the new view is wellcome to be inconsistent does not follow from
> : that. It is written pair lines below. BTW, for tagged types it is
> : different and semantically consistent:
> : 
> : procedure Foo (X : Base'Class) is
> :   XX : Derived renames X;           -- Illegal
> :   XX : Derived renames Derived (X); -- Legal and *CHECKED*
> 
> I'm not sure, considering
> 
>    procedure Foo (X : in out Varying'Class) is
>       subtype Nono is Changeant (false);
>       X1 : Changeant renames X;           -- Illegal
>       X2 : Nono renames Changeant(X); -- Legal and *CHECKED*

You could make it even more scandalous:

   X2 : Nono renames Nono(X); -- !!!No discriminant checks!!!

>    begin
>       X2.messy := 2 * X2.messy;
> -- constraint_error when discriminant is true
>    end Foo;
>  
> where
> package Derivs is
> 
>    type Varying (special: Boolean)
>    is
>      tagged record
>         comp: Natural;
>      end record;
>
>    type Changeant (very_special: Boolean)
>    is
>      new Varying(very_special) with
>       record
>          case very_special is
>             when true =>
>                another: Boolean;
>             when false =>
>                messy: Integer;
>          end case;
>       end record; 
> end Derivs;

Clarification: I meant tags, not discriminants (of possibly tagged types). 
What happens in your example, is the following. Nono has two constraints:

1. The discriminant very_special => False
2. The tag => Changeant'Tag (viewed as a constraint for any of the classes: 
Varying'Class, Changeant'Class)

When you rename X, the constraint 2 is checked. The constraint 1 is silently 
ignored as ARM requires.

Here we are. Now, convince me that it is GOOD.

-- 
Regards,
Dmitry A. Kazakov
www.dmitry-kazakov.de