[issue1979] Make Decimal comparisons with NaN less arbitrary

[Mark Dickinson]

Mark Dickinson added the comment:

Yes, I think it's necessary.

Speaking of standards, it's the current behavior which isn't backed by any 
standard or rationale other than the historical one involving now-defunct 3-
way comparisons.

The proposed behavior is much closer to that specified by C99 (see sections 
7.12.14 and section F.3) and by IEEE 754 and its upcoming revision (see 
section 5.11 of the most recent draft, which is linked to from the wikipedia 
page for IEEE 754r).  Even better from a standards-compliance perspective 
would be to have all comparisons except != involving NaNs raise the 
InvalidOperation flag.

The current behavior also breaks something fundamental:  namely, that x < y 
should have the same truth value as y > x.  Python even depends on this for 
reversing comparison operators, which explains, but in my opinion doesn't 
excuse, the following oddity:

>>> 2 < Decimal("NaN")
True
>>> Decimal(2) < Decimal("NaN")
False

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