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From: "J-P. Rosen" <rosen@adalog.fr>
Newsgroups: comp.lang.ada
Subject: Re: Ada Successor Language
Date: Mon, 25 Jun 2018 14:41:10 +0200
Organization: Adalog
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Xref: reader02.eternal-september.org comp.lang.ada:53296
Date: 2018-06-25T14:41:10+02:00
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Le 25/06/2018 à 10:58, Alejandro R. Mosteo a écrit :
> See this one, which is the case that bit me recently:
> 
> procedure Blah is
>     generic
>         type Something is private;
>     package Duh is       
>         type Meh is new Something;       
>     end Duh;
> 
>     type Oh is tagged null record;
>     
>     package Ouch is new Duh (Oh);
> 
> begin
>    null;
> end Blah;

A private formal allows tagged and untagged types. However, deriving
without extension is legal only for untagged types. Since it is a
specification (that you are supposed to have read), the compiler assumes
the best and allows the derivation. When you instantiate with a tagged
type, you would be doing something illegal and the instantiation is not
allowed.

The same derivation would not compile in the body of the generic, where
the worst is assumed.

-- 
J-P. Rosen
Adalog
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