Union type expressions added in PEP 604 throw an error when both operands are GenericAlias objects.

Eg the following works::

int | list[str]

The following throws TypeError::

list[int] | dict[float, str]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for |: 'types.GenericAlias' and 'types.GenericAlias'

I have submitted a PR to fix this. Coincidentally, it also fixes the fact that union expressions weren't de-duplicating GenericAlias properly.

Eg::
>>> list[int] | int | list[int]
list[int] | int | list[int]

For non-GenericAlias type expressions, the new code shouldn't be much slower. Rich compare is only used for GenericAlias objects. This isn't very scientific, but 

python -m timeit "int | str | float"

# original
1000000 loops, best of 5: 295 nsec per loop

# purely rich compare
1000000 loops, best of 5: 344 nsec per loop

# check  for GenericAlias and rich compare only for that
1000000 loops, best of 5: 297 nsec per loop

There is also a problem with typing module.

>>> typing.List[int] | dict[float, str]
typing.Union[typing.List[int], dict]

@Serhiy, wow interesting find, it seems to be typing's repr problem rather than the actual types itself:

>>> typing.Union[dict[int, str], list[str]]
typing.Union[dict, list]

>>> typing.Union[dict[int, str], list[str]].__args__
(dict[int, str], list[str])

The __args__ seem to be correct, so I'm guessing the typing repr went wrong somewhere. That should be the case for your example too:

>>> alias = typing.List[int] | dict[float, str]
>>> alias
typing.Union[typing.List[int], dict]

>>> type(alias)
<class 'typing._UnionGenericAlias'>

>>> alias.__args__
(typing.List[int], dict[float, str])

I'll work on this. If I don't reply back in a week, someone else is free to take over this issue.

New changeset 1f7dfb277e5b88cddc13e5024766be787a3e9127 by kj in branch 'master':
bpo-42233: Correctly repr GenericAlias when used with typing module (GH-23081)
https://github.com/python/cpython/commit/1f7dfb277e5b88cddc13e5024766be787a3e9127

New changeset e81e09bfc8a29a44a649a962870a26fe0c097cfa by Miss Islington (bot) in branch '3.9':
bpo-42233: Correctly repr GenericAlias when used with typing module (GH-23081)
https://github.com/python/cpython/commit/e81e09bfc8a29a44a649a962870a26fe0c097cfa

New changeset 4eb41d055e8307b8206f680287e492a6db068acd by kj in branch 'master':
bpo-42233: Add union type expression support for GenericAlias and fix de-duplicating of GenericAlias (GH-23077)
https://github.com/python/cpython/commit/4eb41d055e8307b8206f680287e492a6db068acd

Guys, any chance the fix will land in 3.9? It is affected as well.

@Dávid

No it won't land in 3.9. Union type expressions (PEP 604) were only officially added in 3.10. We don't backport new features, only bugfixes. Sorry!

@kj Sorry, for some reason, I thought the that issue affected Union as well, but I have just re-tested it and found that it works.