Feature or enhancement

Proposal:

This was pointed out by Nikita Sobolev in #106246 (comment), it seemed worthwhile to have as a separate issue.

from typing import TypeAlias
# old style
Number: TypeAlias = int | float
assert isinstance(0, Number)  # ✅
# new style
type Number = int | float
assert isinstance(0, Number)  # ❌ TypeError: isinstance() arg 2 must be a type, a tuple of types, or a union

This prevents changing old code to the new PEP 695 syntax if type aliases define unions and are used in isinstance checks.

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

No response

Linked PRs

• gh-113904: Makeisinstance work with PEP695 type aliases #113911