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From: Robert A Duff <bobduff@TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: 'Class'Class'Class'Class ad libitum
Date: Tue, 13 Sep 2016 14:53:15 -0400
Organization: The World Public Access UNIX, Brookline, MA
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Date: 2016-09-13T14:53:15-04:00
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"J-P. Rosen" <rosen@adalog.fr> writes:

> I guess this is allowed because nobody cared about complicated rules to
> forbid it (mainly on the ground that it's useless, but harmless). 

I think it was to make this work:

    generic
       type Formal (<>) is tagged private;
    package G is
       pragma Elaborate_Body;
    end G;

    package body G is
       subtype S is Formal'Class;
    end G;

    with G;
    package P is
       type T is tagged null record;
       package Inst is new G (Formal => T'Class);
    end P;

We don't want the legality of an instance to depend on the contents of
the generic body.

> ...There
> are many things like that:
>   T'(T'(T'(X)))
>   X+0+0+0
>   ...

- Bob