Constrained type param with keyof seems to have inconsistent behavior

[Phryxia]

🔎 Search Terms

generic, keyof, constrained, mapped type, conditional type

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about "This conditional type returns never when it should return the true branch"
• Also inspected Extract not narrowing type union in mapped type passed as generic #57827 Generic constraint wipes out precise key type #56912

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.4.5#code/C4TwDgpgBAYgPAFQDRQNID4oF41QgD2AgDsATAZygGsIQB7AMygSgH5mBtVAXT0JIpRywAE4BLYgHM2UUQFdoALigMAhgBtySqMQgA3CCIBQoSFADiiFKj5EylGvSYJMOBF14E7g4eKkz5bTVNCCMTcGgWHABvKHxlYjkAWwAjQxQQZV8JaQBfMNNoGGBsWCtqWkZmTAB6GqgUujp1CFVicLNzEpxLZAqnaqg6lQ0tIA

💻 Code

type F<T, K> = K extends keyof T ? T[K] extends string ? true : false : never
type G<T, K extends keyof T> = T[K] extends string ? true : false

type T = { x: number, y: string }

type Ft = F<T, keyof T> // boolean
type Gt = G<T, keyof T> // false

🙁 Actual behavior

Ft is boolean while Gt is false

🙂 Expected behavior

I'm not sure, in my opinion the first one (Ft) seems reasonable. But one thing is clear. Their result should be same.

Additional information about the issue

I suspected G<T, K> doesn't count external condition. I think G<T, K> just consider T[keyof T] undecidable, so that just fallb back to falsy type. But I'm not sure why it didn't happend with unconstrained one(F<T, K>)- which conditionally branched.

[MartinJohns]

This is working as intended.

The first is a distributive conditional type. Check the documentation about conditional types. This is unrelated to keyof.

In your G case T[K] results in a union of all property types, so string | number, and this does not extend string, so you end up with false.

In your F case you have a distributive conditional type over K, so you run the conditional type for each property type and end up with a union. So basically you have (string extends string ? true : false) | (number extends string ? true : false). This type resolves to true | false and then is simplified to boolean.

[Phryxia]

@MartinJohns

Thank you for your reply. I think I misunderstood some concept of distributive conditional logic. I'll check it out once more. I'll close this as completed.