mypy should support optional tagged unions

[chdsbd]

Report Type: Bug

Example: https://mypy-play.net/?mypy=latest&python=3.8&gist=529e99849ebe65749d8568b743f21050

from typing import Literal, TypedDict, Union

class NewJob(TypedDict):
    event_type: Literal["new-job"]
    job_description: str

class CancelJob(TypedDict):
    event_type: Literal["cancel-job"]
    job_id: int

request: Union[NewJob, CancelJob, None]

# works
if request is not None:
    if request["event_type"] == "new-job":
        print("Starting new job", request["job_description"])

# fails with `main.py:20: error: TypedDict "CancelJob" has no key 'job_description'`
if request is not None and request["event_type"] == "new-job":
    print("Starting new job", request["job_description"])

• What is the actual behavior/output?
  Mypy does not discriminate TypedDicts when None is part of the Union.
• What is the behavior/output you expect?
  Mypy narrows a Union of TypedDict and None like it does for unions of TypedDicts.
• What are the versions of mypy and Python you are using?
  mypy 0.770
  Python 3.7.6
• Do you see the same issue after installing mypy from Git master?
  I have not tried master.

This is a small issue because there is an easy workaround of checking for None before checking the tag.

[JukkaL]

This looks like a bug.