erroneous type mismatch with `Union` and `Literal` (false positive)

Bug Report

Mypy does not sufficiently simplify Unions and Literals before some type checks (the reproduction case uses assert_type(), but assignments and calls will also trigger the issue). The result is false positive errors.

To Reproduce

[mypy-play.net]

from __future__ import annotations
from enum import (
        Enum,
        auto,
        )
from typing import (
        Literal,
        Union,
        )
from typing_extensions import (
        assert_type,
        reveal_type,
        )

class Color(Enum):
    RED = auto()
    GREEN = auto()
    BLUE = auto()

RedKwds = Literal['RED', 'R']
GreenKwds = Literal['GREEN', 'G']
BlueKwds = Literal['BLUE', 'B']
ColorKwds = Union[RedKwds, GreenKwds, BlueKwds]

RedSpec = Union[Literal[Color.RED], RedKwds]
GreenSpec = Union[Literal[Color.GREEN], GreenKwds]
BlueSpec = Union[Literal[Color.BLUE], BlueKwds]

ColorSpec = Union[Color, ColorKwds]             # ⎱ these are entirely
RGBSpec = Union[RedSpec, GreenSpec, BlueSpec]   # ⎰ equivalent types


rgb_spec: RGBSpec
assert_type(rgb_spec, ColorSpec)  # fails; should be silent             # L34
        # Expression is of type...
        #   Union[
        #       Union[Literal[Color.RED], Literal['RED', 'R']],
        #       Union[Literal[Color.GREEN], Literal['GREEN', 'G']],
        #       Union[Literal[Color.BLUE], Literal['BLUE', 'B']]
        #       ]
        # ... not ...
        #   Union[
        #       Color,
        #       Union[
        #           Literal['RED', 'R'],
        #           Literal['GREEN', 'G'],
        #           Literal['BLUE', 'B']
        #           ]
        #       ]
reveal_type(rgb_spec)                                                   # L50
        # Union[
        #       Literal[Color.RED], Literal['RED'], Literal['R'],
        #       Literal[Color.GREEN], Literal['GREEN'], Literal['G'],
        #       Literal[Color.BLUE], Literal['BLUE'], Literal['B']
        #       ]


# "shaking" the variable (where "shaking" is clearly a no-op) causes
# things to work as expected:
rgb_spec_shaken = (
        rgb_spec
        if isinstance(rgb_spec, Color) else
        rgb_spec
        )
assert_type(rgb_spec_shaken, ColorSpec)  # passes!
reveal_type(rgb_spec_shaken)                                            # L66
        # Union[
        #       Color,
        #       Literal['RED'], Literal['R'],
        #       Literal['GREEN'], Literal['G'],
        #       Literal['BLUE'], Literal['B']
        #       ]

Expected Behavior

The types are provably equivalent; no error should be raised.

Actual Behavior

Mypy does not sufficiently simplify the types, resulting in an erroneous "mismatch".

Your Environment

• Mypy version used: 1.9.0, master 2024-04-22
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.8, 3.12

まず、示されている assert_type() と reveal_type() のケースを使って issue の mypy-play 再現を実行します。特に 34 行目と 50 行目を確認し、その後、それらのチェックに関係する型の単純化および互換性のパスを調べます。完了条件は、同値な Union 型と Literal 型で不一致が発生しなくなり、表示される revealed type が適切に単純化されることです。