Re: Dispatch on the result still does not work?

[Adam Beneschan <adam@irvine.com>]

On Jul 13, 2:47 pm, sjw <simon.j.wri...@mac.com> wrote:
> On Jul 13, 5:55 pm, Adam Beneschan <a...@irvine.com> wrote:
>
> > On Jul 12, 1:30 pm, sjw <simon.j.wri...@mac.com> wrote:
> > > saying 'tag-indeterminate expression must have type "T"(RM 5.2 (6))'
>
> > No, that's not a GNAT problem.  The RM paragraph, which GNAT was good
> > enough to tell you about, specifically says that a tag-indeterminate
> > expression must have type T in this case.  I kind of think that if a
> > GNAT error message refers to a specific RM passage, it's probably a
> > good idea to read that passage before assuming there's a GNAT
> > problem.....
>
> 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.

                                     -- Adam