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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Assignment access type with discriminants
Date: Thu, 23 Mar 2023 20:53:02 +0100
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On 2023-03-23 19:55, Niklas Holsti wrote:

> Perhaps it would be better to make the assignment P := PF2.all'Access 
> illegal, because it in effect converts a constrained access value (PF2) 
> to an unconstrained access subtype (P), and so in some sense violates 
> the prohibition of constrained subtypes of general access types.

Yes this is substitutability violation. Such cases never go without a 
punishment. In this case it is an implementation overhead.

Consider:

   procedure Set (Destination : in out Foo; Source : Foo) is
   begin
      Destination := Source;
   end Set;

The compiler cannot implement Set in a natural way, because Destination 
might be arbitrarily constrained by the caller. E.g. when the actual for 
Destination is P.all. So, the constraint must be passed together with 
the actual. A quite burden.

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