Inheriting class with __slots__ defined

[dhalenok]

• 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

[JukkaL]

The redundant slot definition seems to work at runtime, so this seems pretty low priority.

[dhalenok]

@JukkaL Agree. On a side note, having the redundant slot wastes some memory.