Fix metaclass conflict

[kedder]

This fixes the regression when #88 combined with #89: mypy-1.0 started producing the error:

Metaclass conflict: the metaclass of a derived class must be a
(non-strict) subclass of the metaclasses of all its bases

[kedder]

@euresti please take a look. I had to remove this line to fix errors that started to appear after upgrade to mypy-1.0.

How important do you think this faketi.metaclass_type = iface.metaclass_type is?

[euresti]

Hi. If you remove this line you bring back the caching issue in #86. Basically in the mypy_cache the typeinfo has the metaclass_type but in the fakeinfo it does not. So you'll only see the errors when you rerun the code.

[kedder]

Can you double-check if that is still the case with mypy-1.0? We don't seem to have a regression test for this.

[euresti]

Yeah. Let me double check. Writing a caching regression tests is pretty hard.

[euresti]

Yeah the caching bug is still there:

$ rm -rf .mypy_cache 
$ mypy foo.py iface.py 
Success: no issues found in 2 source files
$ rm -rf .mypy_cache 
$ mypy iface.py 
Success: no issues found in 1 source file
$ mypy foo.py 
foo.py:3: error: Metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases  [misc]
foo.py:6: error: Metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases  [misc]
Found 2 errors in 1 file (checked 1 source file)

[kedder]

Hm, interesting. So on mypy-1.0, instead of hiding the error, #88 actually exposes it with the cold cache?

[kedder]

@euresti can you share those foo.py and face.py?

[euresti]

They are in #86

[euresti]

So a horrible thing we can do is add a ignored_lines (basically a runtime added # type: ignore) for the error that we know is going to be emitted. The only other options are to figure out how to trick check_metaclass_compatibility to not fail the error.

[kedder]

ignored_lines doesn't sound great, because it could just hide other unrelated errors on that line...

[euresti]

True. This failure doesn't have an error code either, it just says misc.

There's another thing that can be done with is to add
impl.metaclass_type = iface.metaclass_type
That will prevent the @implementer from triggering a mypy failure.

But if you subclass the implementer you still have issues.

[kedder]

@euresti I came up with this much simpler and less hacky implementation that seems to fix the caching issue and this new error as well. The mypy-zope tests pass, but to be sure, can you test it on your real codebase?

Also, I removed test_mro_computation_in_forward_reference_to_implementer, because I think it doesn't make sense anymore without this mro tweaking that got removed as well. Correct me if I'm wrong here.

[euresti]

Wow. That worked! I did wonder why we were creating a faketi in the first place.

[kedder]

Released in mypy-zope-0.9.0

[DMRobertson]

Thank you both for tackling the fallout from #89!