[issue11945] Adopt and document consistent semantics for handling NaN values in containers

[Alexander Belopolsky]

Alexander Belopolsky <belopolsky at users.sourceforge.net> added the comment:

> I think it is presently a bug that a list containing
> a NaN value compares equal to itself. 

Moreover, it also compares equal to another list containing the same NaN:

>>> [nan] is [nan]
False
>>> [nan] == [nan]
True

Here is another case of is implies == optimization breaking NaN property in stdlib:

>>> import ctypes
>>> x = ctypes.c_double(nan)
>>> x == x
True

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nosy: +belopolsky

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