[issue29839] Avoid raising OverflowError in len() when __len__() returns negative large value

[Terry J. Reedy]

Terry J. Reedy added the comment:

https://docs.python.org/3/library/functions.html#len * does not specify the exception.  In such cases, we occasionally change exception in x.y.0 releases without prior notice other than News and What's New.  Also, I think unnecessarily exposing a compile-switch dependent internal detail, as now, is almost a bug.  So +1 to applying in 3.7

* The doc does not specify that 'length' cannot be non-negative.  Perhaps it should, so no-one will think that they can hijack '__len__' to return something that is not a length, as usually understood.

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nosy: +terry.reedy

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<http://bugs.python.org/issue29839>
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