From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00
	autolearn=unavailable autolearn_force=no version=3.4.4
Path: 
 eternal-september.org!reader01.eternal-september.org!feeder.eternal-september.org!aioe.org!.POSTED.Oh2LSLzxQ4+YU0Htrufc+A.user.gioia.aioe.org!not-for-mail
From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: Containers, dangling references
Date: Tue, 10 Mar 2020 18:07:25 +0000
Organization: Aioe.org NNTP Server
Message-ID: <ly5zfcaxsy.fsf@pushface.org>
References: <lya74pbhro.fsf@pushface.org>
	<r46iu2$6b2$1@franka.jacob-sparre.dk>
NNTP-Posting-Host: Oh2LSLzxQ4+YU0Htrufc+A.user.gioia.aioe.org
Mime-Version: 1.0
Content-Type: text/plain
X-Complaints-To: abuse@aioe.org
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.3 (darwin)
X-Notice: Filtered by postfilter v. 0.9.2
Cancel-Lock: sha1:9Y8leYQ6tABl5rgpq3IKtWy6jxk=
Xref: reader01.eternal-september.org comp.lang.ada:58190
Date: 2020-03-10T18:07:25+00:00
List-Id: <comp.lang.ada>

"Randy Brukardt" <randy@rrsoftware.com> writes:

> "Simon Wright" <simon@pushface.org> wrote in message 
> news:lya74pbhro.fsf@pushface.org...
> ...
>> with implementation
>>
>>   function Reference (C : aliased in out Container; Position : in Cursor)
>>                      return Reference_Type
>>   is
>>      pragma Unreferenced (C);
>>   begin
>>      return (Element => Position.The_Node.all.The_Element'Access, Dummy => 
>> 1);
>>   end Reference;
>
> The language rules are designed to allow returning part of an aliased
> parameter as the return from a function. But one could also return a
> part of the designated object of a library-level access type.

Is there any difference between an ordinary return & an extended return?
Looking at AARM 6.5(5.8/3), I think not?

> Thus, what matters here is the type declaration of "The_Node", which
> would need to be a library-level access type (or at least
> instance-level) in order of this to work.

There are all at library level.

   type Node_Type is limited record
      The_Element : aliased Element_Type;
   end record;
   type Node_Access is access Node_Type;

   type Container is tagged record
      A_Node : Node_Access;
   end record;
   type Container_Access is access all Container;

   type Cursor is record
      The_Container : Container_Access;
      The_Node : Node_Access;
   end record;

> I note this implementation is missing the check that the cursor is for
> an element in the provided container (and isn't null). Perhaps you
> simplified the implementation for this presentation.

A mixture of this and an incomplete original implementation.

> This sort of expression is a tough one to deal with for
> compilers. Janus/Ada used to (and maybe still does :-) have problems
> with the accessibility of expressions that have multiple access
> types. It would use the wrong type to determine the
> accessibility. Perhaps something similar is happening for GNAT. It's
> also suspicious that the explicit .all changes the result from the
> implicit .all. That suggests a bug more than an intended
> implementation.

That's what I thought.