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From: "Steve Doiel" <nospam_steved94@attbi.com>
Newsgroups: comp.lang.ada
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 <5ee5b646.0202011311.393ce8d9@posting.google.com>
Subject: Re: Valid on QNaN generates exception, compiler bug?
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Thank you Robert.

I was hoping for a response from either you or Tuck (one I can trust).

SteveD

"Robert Dewar" <dewar@gnat.com> wrote in message
news:5ee5b646.0202011311.393ce8d9@posting.google.com...
> "Steve Doiel" <nospam_steved94@attbi.com> wrote in message
news:<cYx68.6829$IZ4.21489@rwcrnsc54>...
> > Interesting.  I expected float_value'Valid to return
> > false, not raise an
> > exception.
>
> And of course your expectation was entirely correct. This
> is a clear bug (some earlier version of GNAT had this bug,
> and for a while we actually installed an implicit exception
> handler to catch the bogus exception and return False, but
> now GNAT does explicit code to test for and catch NaN's
> and infinities. The problem is that a signalling NaN may
> raise an exception just by loading it.
>
> But the prefix of 'Valid is very special. As we read in
> 13.9.2, the ONLY way you can use an invalid value like
> a NaN is in this context:
>
> 12   (20) X'Valid is not considered to be a read of X;
>      hence, it is not an
>      error to check the validity of invalid data.
>
> There is no basis whatsoever for raising a Constraint_Error
> here, the program is correct, and the 'Valid attribute
> should yield False. Report the bug to the relevant vendor.
>
> You definitely do NOT need to guard all 'Valid attributes
> with an exception handler (which is what the previous
> incorrect response would have implied).