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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: GNAT GPL 2013 bug?
Date: Fri, 9 Aug 2013 13:55:40 -0500
Organization: Jacob Sparre Andersen Research & Innovation
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Xref: news.eternal-september.org comp.lang.ada:16732
Date: 2013-08-09T13:55:40-05:00
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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
news:hrkm713gcc7n$.mep6a9ijq09k.dlg@40tude.net...
> On Fri, 9 Aug 2013 03:38:21 -0700 (PDT), sbelmont700@gmail.com wrote:
...
> Now add "tagged" to Q:
>
>   type Q is tagged null record;
>   procedure P (X : access Q) is null;
>
>   Y : access Q := null;
> begin
>   P (Y); -- This is not OK?
>
> I didn't searched ARM for this, but if indeed so, this is a language bug 
> to
> me.

It's not a bug, but a required compatibility wart. In Ada 95, anonymous 
access always excluded null, and the semantics of dispatching require that 
null is excluded from controlling parameters. Requiring explicit writing of 
"not null" on all controlling parameters would have made all existing Ada 95 
code (like Claw) incompatible with Ada 2005, so that could not be 
considered. The only remaining option was the implicit "not null" for 
controlling parameters. It's not pretty, but there was no other choice. (If 
Ada 95 had had "not null" like it should have, this would not have been a 
problem.)

We tried to require Ada 95 compilers to support "not null" syntactically for 
anonymous parameters (with no semantic meaning), so people could write their 
code sensibly, but we were told that we can't change obsolete standards in 
any way.

                                Randy.