Type narrowing on TypedDict with match expresion produces an unexpected error

[rlkandela]

Bug Report

When you try to narrow down the type of a TypedDict by checking with a match expression, it produces an error when it shouldn't. But when you try an equivalent code but instead of using match, using if/else it works as expected.

To Reproduce

from typing import TypedDict, Literal, Union

class EmptyTaskPayload(TypedDict):
    pass

class CheckPendingTaskSystemRebootResponse(TypedDict):
    type: Literal["SYSTEM/REBOOT"]
    payload: EmptyTaskPayload
    
class SystemVersionTaskPayload(TypedDict):
    version: str
    
class CheckPendingTaskSystemVersionResponse(TypedDict):
    type: Literal["SYSTEM/VERSION"]
    payload: SystemVersionTaskPayload

CheckPendingTaskResponse = Union[
    CheckPendingTaskSystemRebootResponse,
    CheckPendingTaskSystemVersionResponse
]

def test_match(pending_task: CheckPendingTaskResponse) -> None:
    match pending_task["type"]:
        case "SYSTEM/REBOOT":
            pass
        case "SYSTEM/VERSION":
            payload = pending_task["payload"]
            version = payload["version"]
#                                      ^ Error produced here
# main.py:28: error: TypedDict "EmptyTaskPayload" has no key "version"  [typeddict-item]

def test_if(pending_task: CheckPendingTaskResponse) -> None:
    if pending_task["type"] == "SYSTEM/REBOOT":
        pass
    elif pending_task["type"] == "SYSTEM/VERSION":
        payload = pending_task["payload"]
        version = payload["version"]

Playground and Gist

Expected Behavior

Expected the code to be absent of errors since it's correct.

Actual Behavior

Produces an error in the index of payload with the key "version" saying that the other type (EmptyTaskPayload) does not have that key. Which is strange as the type system givs payload the correct type (SystemVersionTaskPayload) and in the alternative version with if/else it works.

Your Environment

• Mypy version used: 1.8.0
• Python version used: 3.10.12

[erictraut]

I don't think mypy currently supports discriminated unions of TypedDicts for pattern matching, so this isn't really a bug. It's a reasonable feature request, however. FWIW, pyright supports this feature.

[rlkandela]

Shall I close this issue and open a feature request?

[tmke8]

I think this is the same as #16286. Your issue title is maybe better though.