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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: Ada Annoyances
Date: Fri, 23 Jun 2017 11:25:37 +0100
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Date: 2017-06-23T11:25:37+01:00
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AdaMagica <christ-usch.grein@t-online.de> writes:

> Am Donnerstag, 22. Juni 2017 23:52:42 UTC+2 schrieb Dmitry A. Kazakov:
>> There is many ways to achieve the same [wrong] effect. E.g. without any 
>> dispatch:
>>
>>     procedure Foo (P : Parent) is
>>     begin
>>        Bar (Some_Descendant_Of_Parent (P));
>>     end Foo;
>>
> This code seems illegal to me, at least for tagged types:
> 1. If Foo is a primitive operation, the body cannot know the descendants.
> 2. The type conversion is illegal - here, an extension aggregate is needed.

Illegal as-is because "downward conversion of tagged objects not
allowed".

   procedure Foo (P : Parent) is
   begin
      Child (Parent'Class (P)).Bar;
   end Foo;

will compile (and, presumably, work - but, ugh, unless you are sure
there can't be a constraint error at the Child conversion, and even then
...)