Bug Report

🔎 Search Terms

partial application generic bind return type

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about bind, generics, partial application

⏯ Playground Link

Playground link with relevant code

💻 Code

function a<T extends Array<any>>(anArray: T): T {
    return anArray;
}

let b = a.bind(undefined, [1]); // b has type () => any[]
let c = () => a([1]);           // c has type () => number[]


// Further simplified

function identity<Type>(arg: Type): Type {
  return arg;
}

let d = identity.bind(undefined, "test"); // d has type () => unknown
let e = () => identity("test");           // e has type () => string

🙁 Actual behavior

The types of b and c are different, and the types of d and e are different.

🙂 Expected behavior

I'd expect them to both have the same type (b that of c, () => number[]), d that of e () => string), as the expressions are doing the same thing (arrow function vs bind for partial application of a function).

This limitation is noted in a comment to the "Strict bind, call, and apply methods on functions" PR, but I couldn't find an actual issue referencing this behaviour.
That comment is also mentioned as a known limitation in a similar issue which was closed in #28920, but unfortunately that pr only fixes the issue when binding only this, and not other parameters.