New issue 1974: Different behaviour for collections of nans from CPython
https://bitbucket.org/pypy/pypy/issue/1974/different-behaviour-for-collections-of
David MacIver:
All of the following asserts pass in cpython (I've tried on 2.7, 3.3 and 3.4)
but some of them fail on pypy (I've tried on pypy-2.5.0 and pypy3-2.4.0).
```
#!python
n1 = float('nan')
n2 = float('nan')
x = [n1]
assert n1 in x
assert n2 not in x
assert x.index(n1) == 0
try:
x.index(n2)
assert False
except ValueError:
pass
```
Basically, it seems like in cpython collection methods have a shortcut that
treats x and y as equal if x is y even if not x == y.
The behaviour for sets is also different. For set like collections CPython
treats reference equal nans as equal but other nans as distinct.
```
#!python
n1 = float('nan')
n2 = float('nan')
n3 = float('nan')
x = {n1, n2}
assert len(x) == 2
assert n1 in x
assert n2 in x
assert n3 not in x
t = {n1}
assert t == t
assert t == {n1}
assert t != {n2}
```
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