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From: Adam Beneschan <adam@irvine.com>
Newsgroups: comp.lang.ada
Subject: Re: Dispatch on the result still does not work?
Date: Mon, 13 Jul 2009 08:17:07 -0700 (PDT)
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Date: 2009-07-13T08:17:07-07:00
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On Jul 13, 8:13=A0am, Adam Beneschan <a...@irvine.com> wrote:
> On Jul 11, 3:04=A0am, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
> wrote:
>
>
>
>
>
> > Considering this one:
>
> > package P is
> > =A0 =A0type T is tagged null record;
> > =A0 =A0function Value return T;
>
> > =A0 =A0type S is new T with null record;
> > =A0 =A0function Value return S;
> > end P;
>
> > package body P is
> > =A0 =A0function Value return T is
> > =A0 =A0begin
> > =A0 =A0 =A0 return (null record);
> > =A0 =A0end Value;
>
> > =A0 =A0function Value return S is
> > =A0 =A0begin
> > =A0 =A0 =A0 return (T with null record);
> > =A0 =A0end Value;
> > end P;
>
> > use P;
>
> > =A0 =A0X : T'Class :=3D S'(Value);
> > begin
> > =A0 =A0X :=3D Value; -- Ambiguous?
>
> > I would expect it rather dispatching on the tag of X, i.e. selecting Va=
lue,
> > which returns S.
>
> The problem here (I mean the problem with your program, not a problem
> with the Ada language) is that the overloading rules, which determine
> what the meaning of the identifier "Value" is when there is more than
> one possibility, do not take the dynamic semantics into account. =A0The
> overloading rules basically work like this: We consider all possible
> combinations of all possible meanings of the identifiers, then keep
> only the legal ones (legal according to the "legality rules";

Technically, that should have read 'legal according to the "Name
Resolution Rules"'.  The rules listed as Legality Rules in the RM
aren't even considered here (thus, for example, a procedure call with
two meanings can be ambiguous even if one of them involves passing a
constant as an OUT parameter).  My point was that the rules in the
Dynamic Semantics are not considered at all during this phase.

                                  -- Adam