This is not mypy, it's "python-the-language": the annotations are values, and, in this case, the method annotations are evaluated when encountered - which is before the end of the class. So you are trying to use a value still not defined. It's like doing something like

    def foo() -> Bar:
       ...

    class Bar:
       ...

obviously the python interpreter cannot find a "Bar" definition during the parsing of the foo function

You're saying this is how the language works.

I'm saying the language could work differently.

The case of referring to a class inside its definition is quite different from your "Bar" case. The thing referred to is already declared, it's just not fully defined yet.

There is no logical reason I can see making it impossible for Python to honor the obvious intention of the programmer here. It seems it has just chosen not to do so. Though I'll admit I haven't thought through every weird corner case :)

PEP 563 is intended to resolve it: https://www.python.org/dev/peps/pep-0563/

As rcfox said, you can already enable this behavior, and it will be included in Python 4.0.

The language will recognise it in a future version by default, or a from future import can be used now.

mypy isn't behind (here), it's ahead.