Dict literal of type `A` cannot be assigned to variable of type `A | B` (incompatible type error)

Bug Report

Mypy fails with unions of a union of dict types with literal keys: a dict literal which can be assigned to a variable of type A cannot be assigned to a variable of type A | B.

To Reproduce

Gist: https://mypy-play.net/?mypy=latest&python=3.12&gist=4564762bf0284f05695be7fc30c79020

from typing import Literal

A = dict[Literal["foo"], str]
B = dict[Literal["bar"], int]

a: A = {"foo": "s"} # works
b: B = {"bar": 5} # works

T = A | B
t1: T = {"foo": "s"} # error
t2: T = {"bar": 5} # error

Expected Behavior

It should work without errors.

Actual Behavior

main.py:10: error: Incompatible types in assignment (expression has type "dict[str, str]", variable has type "dict[Literal['foo'], str] | dict[Literal['bar'], int]")  [assignment]
main.py:11: error: Incompatible types in assignment (expression has type "dict[str, int]", variable has type "dict[Literal['foo'], str] | dict[Literal['bar'], int]")  [assignment]

Your Environment

• Mypy version used: 1.11.0
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.12

まず、リンクされた mypy-play の再現コードを使って動作を確認しながら、main.py の最小例に対して mypy を実行します。dict 型のユニオンに代入される dict リテラルの処理を追跡し、両方の代入がエラーなしで通ることを示すカバレッジを追加します。