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

まず、報告された tytest.py の例を mypy --strict で実行し、受け入れられたコードを Python の AttributeError と比較します。ダブルアンダースコア名の処理とインスタンスの初期化に関する型チェックの経路を読み、その後、サブクラスとスーパークラスのプライベート属性を区別し、プログラムの実行前に欠落している属性を報告するテストカバレッジを追加します。