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From: "J-P. Rosen" <rosen@adalog.fr>
Newsgroups: comp.lang.ada
Subject: Re: Assignment access type with discriminants
Date: Thu, 23 Mar 2023 18:04:36 +0100
Organization: Adalog
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Le 22/03/2023 à 10:19, Dmitry A. Kazakov a écrit :
> I stumbled on a curious fact.
> 
> The value of an object with a discriminant can be changed to a value 
> with a different discriminant if the type's discriminants are defaulted.
> 
> Right?
> 
> Wrong! Not through an access type!
> 
(...)
> Is this a compiler bug or intentional language design? Any language 
> lawyers?
> 
An access value is always constrained by its initial value; this is 
necessary because of constrained access subtypes. Here is a slightly 
modified version of your example:

procedure Test is
    type F is (F1, F2, F3);

    type Foo (K : F := F1) is record
       case K is
          when F1 =>
             X1 : Integer;
          when F2 =>
             X2 : Float;
          when F3 =>
             X3 : String (1..2);
       end case;
    end record;
    type Foo_Ptr is access all Foo;
    type Foo_Ptr2 is access Foo;
    X : aliased Foo;
    P : Foo_Ptr := X'Access;
    PF2: Foo_PTR2 (F2);
begin
    X := (F2, 1.0);   -- OK
    PF2 := new Foo (F2);
    P := PF2.all'Access;
    P.all := (F1, 3); -- Error!
end Test;

Without this rule, PF2.all would now designate a value whose 
discriminant is F1!
-- 
J-P. Rosen
Adalog
2 rue du Docteur Lombard, 92441 Issy-les-Moulineaux CEDEX
https://www.adalog.fr https://www.adacontrol.fr