Mypy's `--warn-unreachable` option erroneously considers some `isinstance` calls to be always False with mypy-zope 0.9.0 about mypy-zope HOT 6 OPEN

Hm, but isn't it right though? There is no way for mypy to know if x is an instance of MyFoo. It is an instance of IFoo according to the description (or maybe None). It may be MyFoo, or any other implementation of IFoo.

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There is no way for mypy to know if x is an instance of MyFoo. It is an instance of IFoo according to the description (or maybe None). It may be MyFoo, or any other implementation of IFoo.

I agree with this. But mypy cannot conclude from this that isinstance(x, MyFoo) must be False in all circumstances---which is what I think leads to the unreachable error.

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That's fair. I'll take to look.

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Many thanks! FWIW we've only seen this problem when checking our test, so it's not the end of the world as is!

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It is strange, but I can't repro it - see #92. @DMRobertson are you using some special mypy configuration to trigger the problem?

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Ahh, sorry. I should have double-checked this with a clean environment.

I think you need the --warn-unreachable command line flag to see this behaviour. (I was using Synapse's mypy config which has this turned on by default.)

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