Re: Generic-Package Elaboration Question / Possible GNAT Bug.

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Mon, 21 Nov 2011 08:08:04 -0500, Robert A Duff wrote:

> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:
> 
>>>> I don't know if Ada keeps that promise for generic bodies, ...
> 
> I think it does.  I'd be interested in any counterexamples.

GNAT sometimes reports errors in generic bodies upon instantiation, but I
am not sure if these are not compiler bugs.

>>...Generic contracts are too weak for that.
> 
> Well, it's not that the contracts are too weak.  You just have to
> consider the entire generic spec (including the private part,
> unfortunately), as part of the "contract".

That is C++'s approach. If matching formal parameters does not imply
legality, that does not look very contracted.

> Also, the generic contract model applies only to compile-time rules.

Sure, that is a contract model of the meta language of generics. If it were
properly contracted generics produced only legal "Ada proper" programs.

> The fact that the instance body is guaranteed to be legal doesn't
> mean that it will work properly!

Yes, the same applies to non-generic subprograms. Properly typed arguments
do not guaranty it working. Legality does not imply correctness.

> And there are a few cases where we
> deliberately "cheat" in the RM, by making something raise an exception
> in a generic, when it would normally be a legality rule.

Conversion of errors into exceptions is an extremely bad idea. The same
fault is represented by dynamic pre-/pos-conditions, assertions,
accessibility checks, tag errors.
 
-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de