Signature incompatible with mapping when key is a union of 2 types

Bug Report

I am getting this unexpected error:

overload_mypy.py:15: error: Signature of "__getitem__" incompatible with supertype "dict"
overload_mypy.py:15: note:      Superclass:
overload_mypy.py:15: note:          def __getitem__(self, Union[KeyEnum, ADDITIONAL_PROPERTY]) -> Union[int, str]
overload_mypy.py:15: note:      Subclass:
overload_mypy.py:15: note:          @overload
overload_mypy.py:15: note:          def __getitem__(self, Literal[KeyEnum.PROP_A]) -> int
overload_mypy.py:15: note:          @overload
overload_mypy.py:15: note:          def __getitem__(self, ADDITIONAL_PROPERTY) -> str
overload_mypy.py:15: error: Signature of "__getitem__" incompatible with supertype "Mapping"
overload_mypy.py:15: note:          def __getitem__(self, Union[KeyEnum, ADDITIONAL_PROPERTY]) -> Union[int, str]
overload_mypy.py:15: note:          @overload
overload_mypy.py:15: note:          def __getitem__(self, Literal[KeyEnum.PROP_A]) -> int
overload_mypy.py:15: note:          @overload
overload_mypy.py:15: note:          def __getitem__(self, ADDITIONAL_PROPERTY) -> str
Found 2 errors in 1 file (checked 1 source file)

To Reproduce

run mypy on this code

import enum
import typing
import typing_extensions

class ADDITIONAL_PROPERTY(str):
    pass

class KeyEnum(str, enum.Enum):
    PROP_A = "prop_a"

OUTPUT_BASE_TYPES = typing.Union[int, str]

class SomeDict(typing.Dict[typing.Union[KeyEnum, ADDITIONAL_PROPERTY], OUTPUT_BASE_TYPES]):
    
    @typing.overload
    def __getitem__(self, name: typing_extensions.Literal[KeyEnum.PROP_A]) -> int:
        ...
    
    @typing.overload
    def __getitem__(self, name: ADDITIONAL_PROPERTY) -> str:
        ...
    
    def __getitem__(
        self,
        name
    ):
        # dict_instance[name] accessor
        return super().__getitem__(name)

Expected Behavior

code should pass a mypy check because keys are constrained to strings or an enum
overloads are exhaustive

Actual Behavior

my reports errors

Your Environment

• Mypy version used: mypy 0.971 (compiled: yes)
• Mypy command-line flags: n/a
• Mypy configuration options from mypy.ini (and other config files): n/a
• Python version used: Python 3.9.17 (main, Jun 6 2023, 14:44:03) [Clang 14.0.0 (clang-1400.0.29.202)] on darwin

Start by running mypy 0.971 on the reproduced overload_mypy.py example and inspect the error at line 15. Trace the overload compatibility check for the Union[KeyEnum, ADDITIONAL_PROPERTY] mapping key and the two getitem overloads. Done means the example passes mypy without the reported superclass and Mapping signature errors.