Incorrect type check for nested unions of dicts

Bug Report

For all types X and Y, if a type checks correctly as X it should also check correctly as Union[X, Y].

This is not the case for at least one specific combination of unions and dicts. See reproduction below.

To Reproduce

from typing import Dict, Union

A = Dict[str, Union[str, Dict[str, str]]]
B = Union[A, Dict[str, str]]

example1: A = {"x": {"y": "z"}}
example2: B = {"x": {"y": "z"}}

Playground URL: https://mypy-play.net/?mypy=latest&python=3.11&gist=c72e66a8600324e7b1c1af24a0c533ca

Expected Behavior
example1 and example2 type check as valid.

Actual Behavior

example2 fails to type check with:

main.py:7: error: Incompatible types in assignment (expression has type "Dict[str, Dict[str, str]]", variable has type "Union[Dict[str, Union[str, Dict[str, str]]], Dict[str, str]]")  [assignment]
Found 1 error in 1 file (checked 1 source file)

Your Environment

Python 3.10 and 3.11.
Mypy 1.3.0

まず、提供されている Python の再現コードを実行し、リンクされた mypy playground で 2 つの代入の動作を比較します。dicts を含むネストされた unions に対する型チェックの経路を追跡します。example1 と example2 の両方が、報告された代入エラーなしで型チェックを通れば完了です。