Bug Report

🔎 Search Terms

Generics, Mapped types, key remapping, Array, Tuple

🕗 Version & Regression Information

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

⏯ Playground Link

Playground link with relevant code

💻 Code

type Copy1<T> = {[K in keyof T]: T[K]}

type Copy2<T> = {[K in keyof T as K]: T[K]}

type Test1 = Copy1<[1, 2, 3]> // good - [1, 2, 3]

type Test2 = Copy2<[1, 2, 3]> // bad - { [x: number]: 3 | 1 | 2, toString: () => string, ... }

🙁 Actual behavior

Trivial as K key remapping is causing tuple/array to be converted into an object (yes, same thing for arrays)

🙂 Expected behavior

trivial key remapping should not change the behavior of the mapped type. Tuple/array should not become object since TS 3.1. Copy1 and Copy2 should be equivalent

Notes:
Of course, it's a simplified example. In my actual use case, I had a conditional type like as T extends unknown[] ? K : .... The workaround was to move the conditional type out of the mapped type and write the mapped type twice