std.math.isIdentical and NaN

[Dan Olson via Digitalmars-d]

"Jonathan M Davis" <jmdavisProg at gmx.com> writes:

> On Monday, 22 June 2015 at 08:10:20 UTC, Dan Olson wrote:
>> Docs for isIdentical say:
>>
>>    Same as ==, except that positive and negative zero are not
>> identical,
>>    and two NANs are identical if they have the same 'payload'.
>>
>> However, it returns false for NaN's with different signbits but same
>> payload.  Should this be the case?
>>
>> Ran into this because isIdentical is used in unittests to compare
>> NaN's but I find an occassional test fails due to signbit for some
>> operations on LDC ARM.  I wrote an isNaNWithPayload() predicate for
>> the failing tests, but wonder if isIdentical should ignore signbit
>> for NaNs or have its docs changed.
>
> Well, given the name of the function and the fact that it's clearly
> trying to check for NaN equality rather than treating their comparison
> as always false, I would have expected that it would be true for NaNs
> if they were absolutely identical, which presumably includes the
> signbit, but I really don't know much about how NaNs are implemented,
> so I don't know what the implications of that are. Still, it seems odd
> to compare part of the NaN for equality but not all of it.

Yeah, based on the name "isIdentical", its current behaviour

   assert(!isIdentical(-NaN(0xabc), NaN(0xabc));

does seem to correct.  In that case the documentation could say "NaNs
are identical if they have the same payload and signbit".