Union of unions doesn't work using Python 3.10 Type Union Operator

[Garrett-R]

Bug Report

It seems you can't take a union of unions using the new Type Union Operator.

To Reproduce

1. Run Mypy on this file:

from typing import TypeAlias, Union


class Dog:
    pass

class Cat:
    pass

class Salmon:
    pass

class Trout:
    pass


Mammal: TypeAlias = Dog | Cat

Fish: TypeAlias = Salmon | Trout

Animal: TypeAlias = Mammal | Fish  # type error!!

Animal2: TypeAlias = Union[Mammal, Fish]  # no type error, as expected

Expected Behavior

In PEP-0604, it says "The existing typing.Union and | syntax should be equivalent.". Therefore, I assumed the last 2 lines above would both behave similarly.

Actual Behavior

Mypy throws type error:

error: Unsupported left operand type for | ("object")

Your Environment

• Mypy version used: 0.931
• Mypy command-line flags: <no flags>
• Mypy configuration options from mypy.ini (and other config files): <no config>
• Python version used: 3.10.2
• Operating system and version: Ubuntu 20.04

[KotlinIsland]

Workaround

Put the union on the right if possible

a = int | float
b = str | a  # works
c = a | str  # error: Unsupported left operand type for | ("object")

[DetachHead]

related: #11426

[svenpanne]

Is there some progress on this issue or a better workaround? The trick above doesn't work if both sides of the union are, well, unions themselves, which is exactly what we have in our code. 😭

[Garrett-R]

Woohoo! Thanks for fixing this!