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

Commencez par exécuter la reproduction Python fournie et par comparer le comportement des deux affectations dans le mypy playground lié. Suivez le chemin de vérification des types pour les unions imbriquées contenant des dicts ; c’est terminé lorsque example1 et example2 passent tous deux la vérification des types sans l’erreur d’affectation signalée.