can’t create a type variable with a default on Python < 3.13 (false positive for invalid-legacy-type-variable)

[flying-sheep]

Summary

https://play.ty.dev/1f971569-26a4-4e0b-bfd4-158f9d46fa7f

if TYPE_CHECKING or sys.version_info > (3, 13):
    T = TypeVar("T", default=int)
else:  # runtime on Python < 3.13
    T = TypeVar("T")

outputs

The default parameter of typing.TypeVar was added in Python 3.13 (invalid-legacy-type-variable) [Ln 5, Col 22]

yeah I know, that’s why I only specify it in valid contexts.

Version

0.0.3

[AlexWaygood]

Thanks! Definitely a bug. We need to suppress this error when we can see we're in unreachable code blocks

[sinon]

Is this a bug? If I change the or to an and (or drop TYPE_CHECKING completely) ty detects the correct branch is Never then updating python version in ty.json to 3.14 the Never branches switch places and ty is still happy. 🤔

https://play.ty.dev/e1f55066-d80f-436a-b7dd-5336c6951107

[AlexWaygood]

Ah, good point @sinon.

Hmm, yes, it looks like we've correctly deduced that the first branch is always reachable, because you've used an or rather than an and, @flying-sheep, and TYPE_CHECKING is treated as always-true by type checkers. Because the first branch is always reachable, we'll therefore emit the diagnostic there. You can make ty happy by doing something like this:

import sys
from typing import TYPE_CHECKING

if TYPE_CHECKING:
    # type checkers think typing_extensions is part of the stdlib,
    # so it's fine to import in an `if TYPE_CHECKING` block
    # even if it's not a dependency
    from typing_extensions import TypeVar

    T = TypeVar("T", default=int)
elif sys.version_info >= (3, 13):
    from typing import TypeVar

    T = TypeVar("T", default=int)
else:
    from typing import TypeVar

    T = TypeVar("T")

But that's admittedly a bit verbose. We could consider just switching this specific rule off if we see you're in an if TYPE_CHECKING block?

[flying-sheep]

@sinon, I updated the playground link to show why your suggestion doesn’t work: https://play.ty.dev/1f971569-26a4-4e0b-bfd4-158f9d46fa7f

I of course want to actually use the TypeVar’s default, which wouldn’t exist anymore if you change the logic to and:

Type int does not match asserted type Unknown (type-assertion-failure) [Ln 12, Col 1]

The else branch is there to be executed at runtime and needs to be be ignored at type checking time.

---

We could consider just switching this specific rule off if we see you're in an if TYPE_CHECKING block?

@AlexWaygood yeah, that would work! If type checkers treat typing_extensions as special in a type checking block, they can also treat later feature addition to typing APIs (like default) in a special way.