Document unreachability handling of return NotImplemented
#20561
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There is a special case about unreachability,
return NotImplemented, which we did not until now mention, which we hereby now mention in the documentation of--warn-unreachable.I also add a test to test this claimed behavior thoroughly. The current test tests its desired behavior, in a class, but the actual behavior of mypy is much broader than that, and applies all the time, it seems.
Fixes #20462. That issue is not correct anymore, as it was obsoleted by #20488. However, during its discussion @sterliakov brought up this
NotImplementededge case as something that should be documented. I think the documentation after this PR will be satisfactory by everyone's standards.Similarly, this PR closes #20558.
Theoretically, there could be further special cases of unreachability of which I am not aware, but these can be noted by later PRs if so.