Re: Dispatch on the result still does not work?

[sjw <simon.j.wright@mac.com>]

On Jul 13, 11:50 pm, Adam Beneschan <a...@irvine.com> wrote:
> On Jul 13, 2:47 pm, 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.  The 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
> >    X : P.T'Class := P.T'(P.Value);
> > begin
> >    X := 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.  It will always call
> the Value that returns S.  (Putting the dispatching operation inside a
> type conversion will kill any dispatching---see 3.9.2(17-19).)  And,
> 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 := P.S'(P.Value);
begin
   X := 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).