mypy fails to report missing "private" __x attribute

Mypy doesn't seem to grok that self.__x in a subclass is different from self.__x in a superclass. It also doesn't seem to understand order of initialization.

• Are you reporting a bug, or opening a feature request?

bug

• Please insert below the code you are checking with mypy

class A:
    def __init__(self, x: int) -> None:
        self.__x = x  # replace this with pass and mypy reports the error


class B(A):
    def __init__(self) -> None:
        A.__init__(self, self.__x)


B()

• What is the actual behavior/output?

$ mypy --strict tytest.py 
Success: no issues found in 1 source file
$ python tytest.py 
Traceback (most recent call last):
  File "tytest.py", line 11, in <module>
    B()
  File "tytest.py", line 8, in __init__
    A.__init__(self, self.__x)
AttributeError: 'B' object has no attribute '_B__x

• What is the behavior/output you expect?

mypy should report:

tytest.py:8: error: "B" has no attribute "__x

• What are the versions of mypy and Python you are using?

mypy 0.700

Do you see the same issue after installing mypy from Git master?

yes (mypy-0.770+dev.01ca4e062e38a9db8f462b9464fbbd5dd945a8cf)

• What are the mypy flags you are using? (For example --strict-optional)
  --strict

Start by running the reported tytest.py example with mypy --strict and compare the accepted code with Python's AttributeError. Read the type-checking paths for double-underscore name handling and instance initialization, then add coverage that distinguishes subclass and superclass private attributes and reports the missing attribute before the program runs.