ty should error on Union syntax in function definition without annotations import in Python 3.9 #449
Description
Summary
Splitting this off from #437. Although UnionType was introduced in Python 3.10, it is possible to use the union syntax in function definitions (but not in type aliases or elsewhere) in Python 3.9 if you include from __future__ import annotations
mypy catches this; ty does not.
https://play.ty.dev/5c47f2d5-ef8c-4d2f-b227-f93178b29e57
def f(x: int|str) -> None: ...$ uvx --python 3.9 mypy --strict test.py
test.py:1: error: X | Y syntax for unions requires Python 3.10 [syntax]
Found 1 error in 1 file (checked 1 source file)
$ uvx ty check --python-version 3.9 test.py
All checks passed!https://play.ty.dev/23b020e7-4d07-49d7-9671-9918244dad1c
from __future__ import annotations
def f(x: int|str) -> None: ...$ uvx --python 3.9 mypy --strict test.py
Success: no issues found in 1 source file
$ uvx ty check --python-version 3.9 test.py
All checks passed!Version
ty 0.0.1-alpha.5 (4ad13f2 2025-05-17)