Today we got a bug report in mypy, that Literal[1] | Literal[2] is not supported: python/mypy#11426

Currently this happens on 3.10:

from typing import Literal

Literal[1, 2] | Literal[3]
# error: Unsupported left operand type for | ("object")

This happens because mypy treats | as a regular __or__ operator. And _SpecialForm does not have this method. We have two ways to solve this:

1. Special case it in mypy. This is quite easy, but I don't think that this is a good way
2. Add __or__ method to _SpecialForm type, just like it is done in cpython: https://github.com/python/cpython/blob/14a4fce457033412278ca9a056787db424310dc3/Lib/typing.py#L396-L397 This way we can be sure mypy will just work! 🎉

There are several notes:

• There are _SpecialForms that raise TypeError when used with |. Example: Final | int. But, this happens because Final is not allowed for Union. But, operator is supported. So, I guess this should be handled by a typechecker itself
• TypedDict is not special in typeshed:

  typeshed/stdlib/typing.pyi

  Lines 57 to 58 in f4143c4

  	# TypedDict is a (non-subscriptable) special form.
  	TypedDict: object

  But, | is supported by it:

>>> from typing import TypedDict
>>> class My(TypedDict): ...
... 
>>> My | int
__main__.My | int

• TypeVar is a separate class, not a special form. But, we can use T | S in runtime as well.

I will send a PR with my proposal. We can work from there! 👋