__new__ and subclassing is wrongly interpreted by mypy

[socketpair]

class Base:
    def __new__(cls):
        return 42

    def get(self) -> str:
        return 'hello'

class Child(Base):
    pass


print(Child())
print(Child().get())

This code obviously will fail at runtime:

42
Traceback (most recent call last):
  File "/tmp/qwe2.py", line 13, in <module>
    print(Child().get())
AttributeError: 'int' object has no attribute 'get'

But MyPy sees nothing wrong.

[hauntsaninja]

That's because mypy sees def __new__(cls): as def __new__(cls) -> Any:. If you annotate it as def __new__(cls) -> int:, you'll get an error

[socketpair]

@hauntsaninja why mypy does not infer return type of __new__ ? I think it should

[hauntsaninja]

mypy does not infer unannotated types in general. I agree that it would be useful if mypy was capable of that, see e.g. #4409