Strange behavior using Tagged Union

Bug Report

When using tagged unions, mypy infers correctly with with instance["tag"],
but fails with instance.get("tag")

To Reproduce

1. Write the following code into tagged_union.py:

from typing import Literal, TypedDict, Union


class TaggedTypeA(TypedDict):
    tag: Literal["A"]
    is_a: bool


class TaggedTypeB(TypedDict):
    tag: Literal["B"]


TaggedUnion = Union[TaggedTypeA, TaggedTypeB]

instance_a: TaggedUnion = {"tag": "A", "is_a": True}

if instance_a["tag"] == "A":
    print(f"Am I type a? {instance_a['is_a']}")

if instance_a.get("tag") == "A":
    print(f"Am I type a this time? {instance_a['is_a']}")

2. Run mypy tagged_union.py
3. Get only one error

tagged_union.py:21: error: TypedDict "TaggedTypeB" has no key "is_a"

Error occurs in second if statement

Expected Behavior

I can see that mypy correctly infers the type of instance_a when I use instance_a["tag"] == "A" (the first if expression).

Therefore, I would expect the same behavior when using instance_a.get("tag") == "A" (the second if clause).

Actual Behavior
mypy correctly infers the type of instance_a when I use instance_a["tag"] == "A" (the first if expression).

mypy fails to infer the type when I use instance_a.get("tag") == "A".

In fact, mypy doesn't only not infer the correct type, it infers the opposite type TaggedTypeB which does not make sense at all

Your Environment

• Mypy version used: 0.902
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.8.10
• Operating system and version: Ubuntu 21.04

Start with the tagged_union.py reproduction and run mypy tagged_union.py to compare narrowing for bracket access and get(). Trace the type-narrowing path responsible for TypedDict unions, then add or update a regression test showing that both forms narrow to TaggedTypeA and pass the relevant mypy test suite.