Mypy cannot infer types of initialized variables

I have the following code:

class Service:
    def __init__():
        self.another_service = Service()
     
    def my_method() -> None:
        return self.another_service.another_method_call()

Mypy should gives an error of incompatible types because another_method_call is not returnig None but it is not type checking because it cannot identify what is the type of another_service.

The only way to makes it gives an error is initializing another_service using:

self.another_service: Service = Service() 

I think mypy has enough static information to guess what is the type of another_service wihout needing to someone explicit type it.

No file or test is named in the issue. Start by reproducing the class example with and without the explicit Service annotation, then trace mypy's instance-variable type inference and related tests. Done means mypy infers the initialized attribute type and reports the incompatible return type without requiring the annotation.