Assert on Literal field causes spurious "Non-overlapping equality check"

[intgr]

🐛 Bug Report

Mypy is overzealous in constraining the possible values of a Literal field, assuming that it cannot change after being asserted.

I suppose this can also happen with other types, like Union fields, but in the case of Literal it's particularly awkward.

To Reproduce

from dataclasses import dataclass
from typing import Literal


@dataclass
class Data:
    state: Literal["on", "off"]

    def set_state(self, state: Literal["on", "off"]):
        self.state = state


def test_func():
    obj = Data("on")
    assert obj.state == "on"

    obj.set_state("off")
    #   ^^^^^^^^^^^^^^^^ mutation happens here
    assert obj.state == "off"
    #      ^^^^^^^^^
    # mypy: Non-overlapping equality check (left operand type: "Literal['on']", right operand type: "Literal['off']")

• Mypy version used: mypy 0.780
• Python version used: 3.8.6

[intgr]

Hrm, this is more-or-less duplicate of #9005, although here I'm using Literal instead of Enum.

[hauntsaninja]

Closing as a duplicate of #9005