mypy not flagging subclasses with incompatible constructors

reproducer

from typing import Type
class Foo:
    def __init__(self, a):
        self.a = a

class Bar(Foo):
    def __init__(self):
        super().__init__("Bar")


def takes_foo(foo: Type[Foo]):
    x = foo("Foo")


takes_foo(Foo)
takes_foo(Bar)

observed behavior
mypy raises no errors, but the code raises a TypeError at runtime.

expected behavior
pep 484 states:

when new_user() calls user_class() this implies that all subclasses of User must support this in their constructor signature...A type checker ought to flag violations of such assumptions, but by default constructor calls that match the constructor signature in the indicated base class (User in the example above) should be allowed.

My understanding of the above is that mypy should be raising an error on the Bar definition because its signature is incompatible with that of its superclass. More generally, I tend to be surprised when code that passes mypy raises a TypeError at runtime.

environment
master as of 2019-06-10

$ pipenv graph
mypy==0.710+dev.e2f31ed71bd1edd60bffc86d3fda9da15ba63b3d
  - mypy-extensions [required: >=0.4.0,<0.5.0, installed: 0.4.1]
  - typed-ast [required: >=1.4.0,<1.5.0, installed: 1.4.0]
$ pipenv run python --version
Python 3.7.3

Thank you all for an amazing tool!

Issue の再現例から始め、その動作を PEP 484 のコンストラクターおよびクラスオブジェクトに関する指針と比較してください。mypy がサブクラスのコンストラクターシグネチャをどのように扱うかを調査し、期待される結果が Bar でのエラーなのか、takes_foo(Bar) でのエラーなのかを判断してください。動作が回帰テストでカバーされ、checker が意図した非互換性を報告すれば完了です。