Inheriting class with __slots__ defined

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

Feature request

• Please insert below the code you are checking with mypy.

class Base():
    __slots__: List[str] = ['foo', 'bar']
    if TYPE_CHECKING:
        foo: int
        bar: int

class Derived(Base):
    __slots__: List[str] = ['bar']

• What is the actual behavior/output?

Success: no issues found in 1 source file

• What is the behavior/output you expect?

I believe it would be nice if mypy generated an error on __slots__ declaration in Derived class because 'bar' slot was already declared in Base class.

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

Python 3.8.0, mypy 0.761

まず、slots を使った、提供されている Base と Derived の例を mypy で再現します。継承されたクラス間で型チェッカーが slots をどのように処理するかを追跡し、その後、継承された slot の重複によって診断が生成される一方で、有効な slot 宣言は引き続き受け入れられることを確認します。