Annotated callback parameters are not prioritized above un-annotated ones

[Rugvip]

🔎 Search Terms

"callback inference", "method inference"

🕗 Version & Regression Information

Testing this in the playground it seems to be an issue all the way back to 3.3.3

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.5.4#code/GYVwdgxgLglg9mABFApgZygHgCoD4AUcADrAmgFyIDeAUIosGAIyX5SXYCUiAvLogDc4MACYBuOgzAAmVu0Rde-IaIkBfTpRUjqamjQD0BxADFw0eEiIAnYmkQBaRAHc41gNZoaqDPlr1GFmQlXQAaSUZZRDZKDGsYMABzbj4wmg0JQ2MAWRQoAAs4EXtnGALEAEMwMDgoCtIkYBhrDEcXN09vdCg-COYYxDiE5LSAmTZuKjVwjP0jRFyCopKy-Mrq2vrLRAAbCtanETh0MAByKHaPLt9-KSYJ0alpAaGkyen0zjEgA

💻 Code

function test<T>(options: {
  fn1: (t: T) => void;
  fn2: (t: T) => void;
}): void {}

// Function props - works
test({
  fn1: t => {},
  fn2: (t: string) => {},
});

// Methods with annotation first - works
test({
  fn1(t: string) {},
  fn2(t) {},
});

// Methods with annotation last - doesn't work
test({
  fn1(t) {},
  fn2(t: string) {},
});

🙁 Actual behavior

When providing multiple function properties with the same type parameter as a function parameter, the type parameter will always be inferred from the first function property.

🙂 Expected behavior

I'm expecting the explicitly annotated function callbacks to have higher inference priority than the ones without annotations.

Additional information about the issue

Not sure whether this is a bug or intended behavior, but though I'd report it to check. It seems related to #18654, although turning off strict function types doesn't change the behavior, it just removes the type error where the unknown param is not assignable to string.

It also looks like it's related to for example #58746, but slightly different.

[Andarist]

This one boils down to #47599

Contextual parameters are assigned in source order and it isn't a unique limitation for callbacks contained in objects. The same happens for arguments lists: TS playground

[Rugvip]

@Andarist yep I noticed that too, just found the object form to be a simpler thing to report really 😅

Closing this and adding a 👍 to #47599 instead, thank you! 🙏