>>> import typing
>>> int | typing.T
int | ~T
>>> typing.T | int
typing.Union[~T, int]
>>> T2 = TypeVar('T2')
>>> int | T2
typing.Union[int, ~T2]
>>> T2 | int
typing.Union[~T2, int]

There is a support of TypeVar in type.__or__, but it does not work with user defined TypeVars, only with internal TypeVars defined in the typing module.

The code for recognizing TypeVars must be wrong. Is it also wrong in e.g. list[T]?

Oh this is a fun one :).

> The code for recognizing TypeVars must be wrong. Is it also wrong in e.g. list[T]?

list[T] is correct. I was pretty puzzled since I thought their code is nearly the same, but then I noticed a subtle difference -- GenericAlias checks for type(o).__module__ == "typing", while Union checks for o.__module__ == "typing", and o.__module__ is wherever the user defines it.

Good question. It works correctly in list[T]. There is a tiny difference between implementations of the check in genericaliasobject.c and unionobject.c.

New changeset a158b20019b50e3ece6e4743ec4e6ae8d818b690 by Serhiy Storchaka in branch 'main':
bpo-44632: Fix support of TypeVar in the union type (GH-27139)
https://github.com/python/cpython/commit/a158b20019b50e3ece6e4743ec4e6ae8d818b690

New changeset cc1a47c849a206441c9b370b6ca954862a523082 by Miss Islington (bot) in branch '3.10':
bpo-44632: Fix support of TypeVar in the union type (GH-27139) (GH-27143)
https://github.com/python/cpython/commit/cc1a47c849a206441c9b370b6ca954862a523082