Type narrowing for dict.get("key") is not None

Bug Report

See Code and Explanation below

To Reproduce

from typing import TypedDict, Optional


class DictA(TypedDict):
    my_value: str


class DictB(TypedDict, total=False):
    my_optional_dict: Optional[DictA]


def test(arg: DictB) -> None:
    if arg.get("my_optional_dict") is not None:
        print(arg["my_optional_dict"]["my_value"])

Playground Link

Expected Behavior
According to the TypedDict, we have 3 cases for the Key my_optional_dict:

1. The Dict did not have this Key (indicated by total=False
2. The Value of this Key is None
3. The Value of this Key is a DictA

If we look at the if in the test function, it does 2 things:

1. Check if the Dict contains this Key
2. Check if the value is not None

This means, that the my_optional_dict of arg is always of the type DictA, so mypy should not see a problem here.

Actual Behavior

main.py:14: error: Value of type "Optional[DictA]" is not indexable  [index]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.0.0
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.11.1

Start by running the provided TypedDict reproducer with mypy using the stated Python 3.11 example and no configuration flags. Trace how the type checker narrows arg.get("my_optional_dict") after the None check, then add a regression test based on test() that passes without an indexability error.