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From: dewar@schonberg.cs.nyu.edu (Robert Dewar)
Subject: Re: Valid Attribute and Unchecked Conversion
Date: 1996/10/07
Message-ID: <dewar.844695770@schonberg>#1/1
X-Deja-AN: 187328776
references: <DyCF97.7zp@world.std.com> <1996Oct1.100608.1@eisner>
 <qw6buemy9ea.fsf@gargantua.enst.fr>
 <mheaney-ya023180000110962027510001@news.ni.net> <Dynt6I.5D1@world.std.com>
 <DyqAKA.DJn@thomsoft.com>
organization: New York University
newsgroups: comp.lang.ada
Date: 1996-10-07T00:00:00+00:00
List-Id: <comp.lang.ada>


iKeith says

"        A call to an imported function or an instance of
        Unchecked_Conversion is erroneous if the result
        is scalar, and the result object has an invalid
        representation.

(To answer someone else's question, applying 'Valid directly to the
result of the Unchecked_Conversion is still erroneous.  (Note that
function results are considered (constant) objects in Ada 95, unlike in
Ada 83 where they're just values; one ramification is that you can now
rename a function result.  (But I digress.)))"


Keith is right, this is in the RM, but I think the RM is definitely wrong
here. I take part of the blame, I simply did not notice this particular
mistake. Obviously for scalar types, unchecked conversion (where the sizes
are the same) should not result in erroneous behavior, and a subsequent
call to 'Valid should also be non-erroneous.

I simply missed this because it is in the wrong section (not under UC).
Anyway, certainly GNAT behaves in a sensible manner here, and hopefully
all other Ada 95 compilers will too, so this mistake should have only
very limited impact.