Type Task.perform as a property

[johanrd]

perform is bound in the constructor (this.perform = this._perform.bind(this)), but typed as a method. So typescript-eslint's unbound-method reports {{on "click" this.task.perform}} when type-aware rules run on templates (emberjs/rfcs#1045 comment).

Breaking (types only): as a property, perform's args are checked contravariantly, so a Task<void, [string]> no longer fits Task<unknown, unknown[]>. That assignment was unsound: it allowed perform(123) on a task that takes a string. Code that means "any task" can use Task<any, any[]>, which still fits. The type tests pass.

Non-breaking alternative, if you prefer: type the property through a method signature, as @types/react does for event handlers. The args stay bivariant, so assignability does not change, and unbound-method still sees a property:

perform: { bivarianceHack(...args: Args): T }['bivarianceHack'];

Cowritten by Claude

[machty]

I don't know what "contravariantly" means, mainly I just want to know "is this likely to bite anyone" for the 99% use caes of ember-concurrency, then I'll merge.

[johanrd]

First: it only affects TypeScript compilation, never runtime, and only code that stores a task in a type that accepts any arguments, like Task<unknown, unknown[]>.

I asked claude to find public TypeScript projects that use ember-concurrency (boxel, ember-power-select, hashicorp/design-system, ember-bootstrap, …), and of 13, 12 are unaffected. One, eclipse-pass/pass-ui, uses Task<unknown, unknown[]> as "any task" in 5 places. passing task(async (page: number) => …) there would become a type error. Their fix is one word: Task<unknown, any[]>, which honestly is more accurate: unknown[] never checked the args anyway, since the method type let any task in and allowed perform(123) on a task that takes a string.

So 99% is maybe a stretch. It can break compilation, so maybe bump as a major? (integers are cheap, etc)

[machty]

I think safe to release as minor. In case major issues i'll revert and publish a new minor and then push major with the fix.

[machty]

https://github.com/machty/ember-concurrency/releases/tag/5.3.0