Key in mapped variadic tuple should not be literal after the variadic part

[Josh-Cena]

Bug Report

🔎 Search Terms

Variadic tuple mapped type keyof

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

Playground link with relevant code

💻 Code

type Keys<O extends unknown[]> = { [K in keyof O]: K };

type A = Keys<[string, ...string[]]>;
type B = Keys<[string, ...string[], number]>;

🙁 Actual behavior

A and B both have the variadic part typed as ..."1"[], which is wrong. B is further wrong because the number part becomes "2", while in practice it would rarely be "2".

🙂 Expected behavior

A should be ["0", ...number[]]; B should be ["0", ...number[], number]

[ahejlsberg]

I don't think this is a bug. Consider:

type Box<T> = { value: T };
type Boxified<T> = { [P in keyof T]: Box<T[P]> };

type T0 = Boxified<[number, string, ...boolean[]]>;  // [Box<number>, Box<string>, ...Box<boolean>[]]

With the proposed change, P would be number instead of "2" for the rest element, which means the resulting rest element would be ...Box<string | number | boolean>[] instead of just ...Box<boolean[]>. Not only is that less correct, it is a breaking change. Effectively, number is a less precise way of expressing "element at index 2 or above", whereas "2" gets much closer. This speaks to the fact that in homomorphic mappings over array and tuple types, the key types are just exemplars and not an exact representation of some specific property.

[Josh-Cena]

I also thought about that. The problem is it's really hard to be correct for both the key type itself, and for homomorphic mapped types to continue be homomorphic. However, accessing T[P] and P directly should be two orthogonal actions. In my actual case, I'm doing Entries instead of Keys:

export type Entries<O extends unknown[]> = { [K in keyof O]: [K, O[K]] };

In an ideal world, I'd like Entries<[number, string, ...boolean[]]> to be [["0", number], ["1", string], ...[number, boolean][]]. Does that sound reasonable?

If we have subtracted types, the index should rather by Exclude<number, 0 | 1>, but since we are not there yet, I'd be more happy with number than 2 as the index type.

[typescript-bot]

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.

[ahejlsberg]

@Josh-Cena FWIW, I now agree with your observations and suggested changes. See #57031. Better late than never! 😄