Re: generic formal object of class-wide type

[Matthew Heaney <matthew_heaney@acm.org>]

"David C. Hoos, Sr." <david.c.hoos.sr@ada95.com> writes:

> Matthew Heaney wrote in message ...
> >When I try to compile package Q (see below), I get this error:
> >
> >q.ads:3:25: type of actual does not match type of "O"
> >
> >Why?
> >
> The actual needs to be explicitly converted to the type of the
> formal, like so:
> 
> package Q is new GQ (P.T'Class (P.C.O));

Yes, but why?

If I have a subprogram with a parameter of a class-wide type, like this:

procedure Op (O : T'Class);

and NT is a type in T'Class, then I can do this:

declare
  O : NT;
begin
  Op (O);
end;


No conversion to type T'Class is required in order to call Op.

It is inconsistent that a type conversion is required in order pass the
object as a generic actual.  I'd like to know the specific rule, and a
rationale for the inconsistency.