Detect undefined attribute in child class (when defined in parent)

mypy does not detect an undefined attribute in a child class if it is defined in a parent class.

Given the following code, where the __init__ of a child class forgets to call super().__init__:

class Base():
    def __init__(self, x: int) -> None:
        self.x = x
    def whatsx(self) -> int:
        return self.x

class Child(Base):
    def __init__(self, x: int) -> None:
        pass
        # Note: no call to super(), nor defining self.x.

c = Child(3)
print(c.whatsx())  # mypy does not catch undefined c.x

The real issue is obviously that the child class does not define the x attribute, failing Liskov.

Perhaps this is out-of-scope for mypy, but since programmers make stupid mistakes like this (/me raises hand), it would be awesome if mypy could warn about this error, making said programmers look less stupid ;)

Start by running mypy on the provided Base and Child example, focusing on the child init, the inherited x attribute, and the missing super().init call. Trace how mypy handles inherited attributes and constructor overrides. Done means mypy reliably warns about the missing initialization without incorrectly rejecting valid subclasses.