Running mypy --strict-optional on the mypy codebase itself, we get this error:

mypy/build.py: note: In member "wrap_context" of class "State":
mypy/build.py:1170: error: Yield value expected

The function in question looks like this:

    @contextlib.contextmanager
    def wrap_context(self) -> Iterator[None]:
        save_import_context = self.manager.errors.import_context()
        self.manager.errors.set_import_context(self.import_context)
        try:
            yield
        except CompileError:
            raise
        except Exception as err:
            report_internal_error(err, self.path, 0)
        self.manager.errors.set_import_context(save_import_context)
        self.check_blockers()

This is pretty sensible idiomatic code. Arguably there might be some other appropriate annotation to write for the return type; bracketing that question, it looks to be perfectly well-typed code that shouldn't produce an error. It's also not immediately clear to me how to work around it, except I guess with a # type: ignore.

I suspect this is related to the complicated treatment we give in some places to None as a return type -- cf #1509, #299, #359 -- and that treating None in a more regular way as discussed in #1278 and #1847 would fix it.

It may also be related to #1933, but I haven't yet read through that discussion to understand what's going on there.