Sometimes, `keyof any` is `string`

[AFatNiBBa]

🔎 Search Terms

keyof any string

🕗 Version & Regression Information

Tested in the playground on "v3.3.3333" and "Nightly", it occurs in both versions

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.8.0-dev.20250217#code/FAFwngDgpgBA8gIwFYwLwwN4wNoGsYCWAdjLlGAPYBmMAhkWALoBcMRArgLYJQBOMAX2AB6YTHEA9APzBQkWAGlyAZzSly1eMgDcIsZJnAAJlADGAG1q9YpikWUgYFZK0RJdzpNgDKYbhXMAOgIQPloQCl5GGFEYAAoAFQALAlUEa1pcZQBKIA

💻 Code

type Obj = { [k in keyof any]: number }
//   ^? type Obj = { [x: string]: number; }

type Keys = keyof Obj;
//   ^? type Keys = string | number | symbol

declare const obj: Obj;
obj[Symbol.iterator] // (This breaks)

🙁 Actual behavior

The Obj type allows indexing only with strings

🙂 Expected behavior

The Obj type should allow indexing with every type of property key

Additional information about the issue

No response

[mkantor]

I came across a relevant comment that may be of interest to anyone looking at this issue.

[RyanCavanaugh]

Seems dangerous to change this, based on the PR results, and it's not super clear what the upside is. You can (and probably should?) write type Obj = { [k: string | number | symbol]: number } instead

[AFatNiBBa]

Here's something more similiar to what's actually happening to me

type A<T> = { [k in keyof T]: 1 };

declare function iDontKnow<T>(a: T): [ A<T>, keyof T ];

function wrap<T>(a: T) {
    const [ o, k ] = iDontKnow(a);
    return o[k];
}

type a = { a: number, b: string };
const b: a = { a: 1, b: "ciao" };

const [ o1, k1 ] = iDontKnow({ a: 1, b: 2 });
const v1 = o1[k1]; // Ok: The type argument is inferred

const [ o2, k2 ] = iDontKnow<{ a: number, b: number }>({ a: 1, b: 2 });
const v2 = o2[k2]; // Ok: The type argument is passed explicitly

const v3 = wrap<any>({ a: 1, b: 2 }); // Ok: The type argument is passed through a generic parameter

const [ o4, k4 ] = iDontKnow<any>({ a: 1, b: 2 });
const v4 = o4[k4];
//            ↑ Type 'symbol' cannot be used as an index type. (TS2538)

Isn't it just wrong?

[typescript-bot]

This issue has been marked as "Not a Defect" and has seen no recent activity. It has been automatically closed for house-keeping purposes.

[AFatNiBBa]

I'd like to get a response on this issue, since it has been closed without providing any.
I'm sorry, but I really don't get why this is "Not a Defect": The same operation, with the same arguments can give two different results and one of them is wrong

[AFatNiBBa]

If the problem is still not clear, here's a another example

type A<T> = { [k in keyof T]: 1 };

declare function iDontKnow<T>(a: T): [ A<T>, keyof T ];

const something: any = { a: 1, b: 2 };
const [ o, k ] = iDontKnow(something);
const v = o[k]; // 
//          ↑ Type 'symbol' cannot be used as an index type. (TS2538)