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From: sjw <simon.j.wright@mac.com>
Newsgroups: comp.lang.ada
Subject: Re: Dispatch on the result still does not work?
Date: Tue, 14 Jul 2009 14:06:42 -0700 (PDT)
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Date: 2009-07-14T14:06:42-07:00
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On Jul 13, 11:50=A0pm, Adam Beneschan <a...@irvine.com> wrote:
> On Jul 13, 2:47=A0pm, sjw <simon.j.wri...@mac.com> wrote:

> > OK, mea culpa. However .. if I say P.S'(whatever) and the compiler
> > tells me it's tag-indeterminate my first reaction is that it must be
> > confused!
>
> Yeah, I can understand why this could be confusing. =A0The qualifier
> P.S' doesn't say that the expression in parentheses will (dynamically)
> have type P.S; it says that the operand must resolve to be of the type
> P.S, which, I believe, implies that the expression's type will
> actually be P.S **except** when dispatching is involved.
>
> > If I hadn't been too taken with the symmetry I'd have realised that I
> > should have written
>
> > procedure Q is
> > =A0 =A0X : P.T'Class :=3D P.T'(P.Value);
> > begin
> > =A0 =A0X :=3D P.T'Class (P.S'(P.Value));
>
> > (which (I think) is the 'dynamically tagged' alternative in 5.2(6)
> > which wasn't mentioned by GNAT).
>
> I think this will work, but it will not dispatch. =A0It will always call
> the Value that returns S. =A0(Putting the dispatching operation inside a
> type conversion will kill any dispatching---see 3.9.2(17-19).) =A0And,
> in this case, it will result in a Constraint_Error at runtime since X
> is now constrained to values of type T.

Exception name: CONSTRAINT_ERROR
Message: dmitry.ada:26 tag check failed

I was entertained by this:

   X : P.T'Class :=3D P.S'(P.Value);
begin
   X :=3D P.T'(P.Value);

where P.T'(P.Value) dispatched to the S version (in other words the
actual (dynamic?) type of the target was controlling).