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,FREEMAIL_FROM
	autolearn=unavailable autolearn_force=no version=3.4.4
X-Received: by 2002:a02:9912:: with SMTP id r18mr4221904jaj.2.1547306138689;
        Sat, 12 Jan 2019 07:15:38 -0800 (PST)
X-Received: by 2002:a9d:6f14:: with SMTP id n20mr85245otq.2.1547306138578;
 Sat, 12 Jan 2019 07:15:38 -0800 (PST)
Path: 
 eternal-september.org!reader01.eternal-september.org!feeder.eternal-september.org!news.gegeweb.eu!gegeweb.org!usenet-fr.net!proxad.net!feeder1-2.proxad.net!209.85.166.216.MISMATCH!q69no194408itb.0!news-out.google.com!v71ni274ita.0!nntp.google.com!k10no194493itk.0!postnews.google.com!glegroupsg2000goo.googlegroups.com!not-for-mail
Newsgroups: comp.lang.ada
Date: Sat, 12 Jan 2019 07:15:38 -0800 (PST)
In-Reply-To: <lymuo619yh.fsf@pushface.org>
Complaints-To: groups-abuse@google.com
Injection-Info: glegroupsg2000goo.googlegroups.com;
 posting-host=2003:da:3712:9e01:d250:99ff:fe54:d2d2;
 posting-account=UO7etwoAAAD8dZ9XMqp9pq9hGTWlWnZb
NNTP-Posting-Host: 2003:da:3712:9e01:d250:99ff:fe54:d2d2
References: <167dc83d-daac-49eb-ba79-48866ccde39d@googlegroups.com>
 <lymuo619yh.fsf@pushface.org>
User-Agent: G2/1.0
MIME-Version: 1.0
Message-ID: <0c56d9f4-8861-4c74-b170-a973e3789b08@googlegroups.com>
Subject: 
 =?UTF-8?Q?Re=3A_Overloading_operator_=E2=80=9C=3D=E2=80=9D_for_anonymous_acces?=
	=?UTF-8?Q?s_types=3F?=
From: daicrkk@googlemail.com
Injection-Date: Sat, 12 Jan 2019 15:15:38 +0000
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Xref: reader01.eternal-september.org comp.lang.ada:55272
Date: 2019-01-12T07:15:38-08:00
List-Id: <comp.lang.ada>

On Saturday, January 12, 2019 at 10:50:17 AM UTC+1, Simon Wright wrote:
> daicrkk@googlemail.com writes:
>=20
> > I am working my way through Barnes' excellent Ada book. This is a code
> > sample for deep comparison of linked lists from section 11.7:
> >
> >
> > type Cell is
> >   record
> >     Next: access Cell;
> >     Value: Integer;
> >   end record;
> >
> > function "=3D" (L, R: access Cell) return Boolean is
> > begin
> >   if L =3D null or R =3D null then    -- universal =3D
> >     return L =3D R;                 -- universal =3D (Line A)
> >   elsif L.Value =3D R.Value then
> >     return L.Next =3D R.Next;       -- recurses OK (Line B)
> >   else
> >     return False;
> >   end if;
> > end "=3D";
> [...]
> > I did my best to find anything in the AARM, especially section 4.5.2,
> > but could not make any sense of it.
>=20
> Given ARM 4.5.2(9.1 ff),
>=20
>    At least one of the operands of an equality operator for
>    universal_access shall be of a specific anonymous access type. Unless
>    the predefined equality operator is identified using an expanded name
>    with prefix denoting the package Standard, neither operand shall be
>    of an access-to-object type whose designated type is D or D'Class,
>    where D has a user-defined primitive equality operator such that:
>=20
>    * its result type is Boolean;
>=20
>    * it is declared immediately within the same declaration list as D or
>      any partial or incomplete view of D; and
>=20
>    * at least one of its operands is an access parameter with designated
>      type D.
>=20
> I'm not at all clear why the example code is legal, or why it would be
> legal to call it; since 'access Cell' appears to match "neither operand
> shall be of an access-to-object type whose designated type is D or
> D'Class, where D has a user-defined primitive equality operator ..."
>=20
> Might explain why compiling this example with GNAT (CE 2018) results in
> stack overflow.

I second that. Access Cell is an access-to-object type whose designated typ=
e is Cell (check), Cell has a user-defined primitive equality operator (che=
ck) such that its result type is Boolean (check), it is declared immediatel=
y within the same declaration list as Cell (check), at least one of its ope=
rands is an access parameter with designated type Cell (both operands are, =
check).
According to 4.5.2, universal_access "=3D" should never be allowed to kick =
in at all here, not even with "L =3D null". Or am I missing something?