Re: Longstanding GNAT bug, bad code, or both -- Answer..Bad code.

["Randy Brukardt" <randy@rrsoftware.com>]

This is old, but some enlightenment is provided...

"Stephen Leake" <stephen_leake@acm.org> wrote in message
news:mailman.6.1091897074.414.comp.lang.ada@ada-france.org...
> "Jeff C," <jcreem@yahoo.com> writes:
> > Ok. I submitted a bugzilla report and it has already been
> > cancelled.  Apparently, declaring a variable as aliased makes it
> > contrained.  (According to one of the gcc Ada maintainers)

One would think that the attempt to modify the object inside the procedure
would cause Constraint_Error to be raised. Or was this non-Ada code?

> > After thinking about it, it is probably why the rule was created
> > since I suppose otherwise the alias detection/contraint checking
> > would get pretty hard and potentially computationally expensive at
> > run time...
>
> Sounds reasonable to me. The Annotated reference manual sheds no light
> on this.

If you really want to understand this, read AI-363
(http://www.ada-auth.org/cgi-bin/cvsweb.cgi/AIs/AI-00363.TXT). The reason
for the rules have to do with access subtypes. Tucker says it best: "Once I
sat down and began to piece together the set of patches we made to
try to fix the problems with 3.6(11), I was somewhat appalled." (And I would
have left out the "somewhat"! :-)

Anyway, Ada0Y is going to have a bit of incompatibility in this area, making
various access subtypes illegal, and then will drop 3.6(11). That means
"aliased" will no longer have the bizarre side-effect of making things
constrained. (Janus/Ada never got this right anyway, so I'm happy. :-) So
this issue is fixed (it is unfortunate that we didn't get this right in Ada
95).

                      Randy.