Union of Literals as key in union of TypedDict false positive

Bug Report
The function

def get_value(key: CombinedDictKey, mapping: CombinedDict):
    return mapping[key]

raises an error when mapping is a union of two TypedDicts and key is a union of two Literals containing the keys of those TypedDicts. It works when typed with just one of the Literals and TypedDicts.

To Reproduce

from typing import Literal, TypedDict, Union

class DictA(TypedDict):
    x: int
    y: int

class DictB(TypedDict):
    z: str

class DictAB(DictA, DictB):
    pass

DictAKey = Literal["x", "y"]
DictBKey = Literal["z"]
DictABKey = Union[DictAKey, DictBKey]

dict_a = DictA(x=1, y=2)
dict_b = DictB(z="z")
dict_ab = DictAB(**dict_a, **dict_b)  # type: ignore  # Unrelated issue #11108

def get_value_from_A(key: DictAKey, mapping: DictA):
    return mapping[key]  # Passes type check as expected

def get_value_from_B(key: DictBKey, mapping: DictB):
    return mapping[key]  # Passes type check as expected

def get_value_from_AB(key: DictABKey, mapping: DictAB):
    return mapping[key]  # Fails type check with error: TypedDict key must be a string literal

Expected Behavior
return mapping[key] passes typing in all three cases.

Actual Behavior
In the case of get_value_from_AB, return mapping[key] raises error: TypedDict key must be a string literal; expected one of ("z", "x", "y"). This is unexpected as it passes for the functions typed with only one of DictA or DictB.

Your Environment

• Mypy version used: 0.991
• Python version used: 3.11.0

Start by running the provided Python 3.11 reproduction against mypy, then trace TypedDict key validation for a union of Literal keys and TypedDict mappings. Done means get_value_from_AB type-checks without the false positive while the single-dictionary cases remain passing.