Re: Language lawyer question: Equality on 'Access attributes

["Robert I. Eachus" <rieachus@comcast.net>]

Robert A Duff wrote:

> This is a subtle area, so I could be missing something, but I agree with
> your (Adam's) analysis here.  I think GNAT has a bug.

And I think that GNAT is right, which is why we have the ARG. ;-)  But 
in this case I think this is a low priority AI, and if one is created, I 
would vote for whatever it says just to close the issue.

The technical question is how many "=" operations could apply in this 
case.  My contention was that there are at least two, one associated 
with the 'Access attributes (actually I think there is one for each 
attribute) and one declared by the user.

It is much easier to see why there is an issue if X is on the stack, and 
Y is in a user managed heap, or even just one object at library level, 
and one on the stack.  (That way one value has a nested accessability 
level to be checked, and the other has a static lifetime longer than the 
program.)  You could argue that those pointers can never compare 
equal--and I'd agree.  But the compiler conceptually has two objects of 
different types.  If you want the comparison to occur, you can assign 
one or both to a local access variable, which will "scrub off" the 
access level information.

Yes, the compiler can be smart about this and conclude that if the two 
objects have different accessability levels, they can't compare equal. 
But normally in Ada we prefer to have the compiler tell you there is a 
potential issue rather than have different compilers do different 
things.  (Which is why if this becomes an AI, I will vote back the 
consensus, whatever it is.  It is more important to have a consistent 
language than to "win" language lawyer debates in the ARG. ;-)

-- 
                                           Robert I. Eachus

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