Re: Dispatch on the result still does not work?

[Adam Beneschan <adam@irvine.com>]

On Jul 13, 8:13 am, Adam Beneschan <a...@irvine.com> wrote:
> On Jul 11, 3:04 am, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
> wrote:
>
>
>
>
>
> > Considering this one:
>
> > package P is
> >    type T is tagged null record;
> >    function Value return T;
>
> >    type S is new T with null record;
> >    function Value return S;
> > end P;
>
> > package body P is
> >    function Value return T is
> >    begin
> >       return (null record);
> >    end Value;
>
> >    function Value return S is
> >    begin
> >       return (T with null record);
> >    end Value;
> > end P;
>
> > use P;
>
> >    X : T'Class := S'(Value);
> > begin
> >    X := Value; -- Ambiguous?
>
> > I would expect it rather dispatching on the tag of X, i.e. selecting Value,
> > 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.  The
> 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