Different behavior for Optional with if condition not Var and Var is not None #7696

from typing import List, Optional, Any

class MyClass1:

    _result: Optional[List[str]] = None
    _foo: Any = None

    @classmethod
    def get(cls, some_arg: Optional[str]) -> List[str]:
        if cls._result is None:
            cls._result = [some_arg or 'a'] + (
                cls._foo.split(',') if cls._foo else []
            )
        return cls._result

class MyClass2:

    _result: Optional[List[str]] = None
    _foo: Any = None

    @classmethod
    def get(cls, some_arg: Optional[str]) -> List[str]:
        if not cls._result:
            cls._result = [some_arg or 'a'] + (
                cls._foo.split(',') if cls._foo else []
            )
        return cls._result
#       ^ [mypy] : Incompatible return value type (got "Optional[List[str]]", expected "List[str]")

$ python -m mypy --version
mypy 0.740+dev.742d33a41271206d6eb7dd2e4b04659de4683597

$ python --version
Python 3.7.4