panic: `Option::unwrap()` on `None` in Query::get_attributes for a non-generic `classproperty` class

[renz011tzar]

Summary

Query::get_attributes panics when a class named classproperty or cached_classproperty has no type arguments. The match arm keys on the class name alone, then assumes type arguments are present:

pyrefly/pyrefly/lib/query.rs

Lines 1144 to 1148 in d5ce6d8

	Type::ClassType(c)
	if c.name() == "classproperty" || c.name() == "cached_classproperty" =>
	{
	let result_ty = c.targs().as_slice().first().unwrap();
	(Some(String::from("property")), result_ty)

Type::ClassType(c)
    if c.name() == "classproperty" || c.name() == "cached_classproperty" =>
{
    let result_ty = c.targs().as_slice().first().unwrap();
    (Some(String::from("property")), result_ty)
}

A plain, non-generic classproperty — a common Python idiom for combining classmethod and property — reaches this arm with empty targs, and .first().unwrap() panics.

thread '...' panicked at pyrefly/lib/query.rs:1147:66:
called `Option::unwrap()` on a `None` value

Reproduction

Verified against main @ d5ce6d8. Python input:

class classproperty:
    def __init__(self, f):
        self.f = f
    def __get__(self, obj, owner):
        return self.f(owner)

class Config:
    @classproperty
    def name(cls) -> str:
        return "cfg"

As a test in pyrefly/lib/test/query.rs:

#[test]
fn repro_non_generic_classproperty_panics() {
    let tdir = TempDir::new().unwrap();
    let file_path = tdir.path().join("main.py");
    let code = r#"<the Python above>"#;
    fs_anyhow::write(&file_path, code).unwrap();

    let query = create_query();
    let module_name = ModuleName::from_str("main");
    let path = ModulePath::filesystem(file_path.clone());
    let errors = query.add_files(vec![(module_name, path.clone())]);
    assert!(errors.is_empty(), "unexpected errors: {errors:?}");

    // panics at query.rs:1147
    let _ = query.get_attributes(module_name, path, "Config");
}

Control: changing the class to class classproperty[T, R]: makes the same test pass, isolating the failure to the no-type-arguments case. pyrefly check on the same file reports 0 errors — this affects the Query::get_attributes API rather than the CLI check path.

Note on verification

Found while evaluating Rust verification tooling. Working this through the midas-lex tool identifies the spec owner here as a precondition: indexing element 0 requires the slice to be non-empty, and matching on the class name does not establish that. Modelling the same obligation in Verus reports precondition not satisfied on the indexing as written; the identical body verifies once targs.len() > 0 is available, or with an explicit length check (3 verified, 1 errors).

Suggested fix

Handle the no-type-arguments case rather than indexing unconditionally — for example falling back to ty so the class is treated as a plain attribute:

Type::ClassType(c)
    if c.name() == "classproperty" || c.name() == "cached_classproperty" =>
{
    match c.targs().as_slice().first() {
        Some(result_ty) => (Some(String::from("property")), result_ty),
        None => (Some(String::from("property")), ty),
    }
}

Happy to send a PR if that's welcome — noting CONTRIBUTING asks to discuss approach on an issue first.

[yangdanny97]

the query API is undocumented ATM and till now has only been used for Meta-internal use cases, the surface is not as mature/stable as the rest of the type checker so I'm not surprised there are edge cases

I think your approach could work, another option would be "(None, ty)"

[yangdanny97]

feel free to make a PR

[renz011tzar]

Thanks — (None, ty) makes sense. I'll put up a PR that moves the type-argument check into the match guard, so a classproperty with no type arguments falls through to the existing _ => (None, ty) arm.

#claim

[github-actions]

@renz011tzar you've claimed this issue — it's now assigned to you. Thanks for picking it up! 🎉