"in keyof" type mapping incorrectly interacts with indexers

[LoganGerber]

Bug Report

🔎 Search Terms

Indexer
Type mapping
Type resolution
Typing

🕗 Version & Regression Information

All versions available on the Typescript workbench.

⏯ Playground Link

Playground link with relevant code

💻 Code

// Matches to the names of all non-optional properties in T
type RequiredMembers<T extends object> = {
    [K in keyof T]?: undefined extends T[K] ? never : K;
}[keyof T];


type WorkingExampleType = {
    requiredString: string;
    requiredNumber: number;
    optionalBoolean?: boolean;
};

let x: RequiredMembers<WorkingExampleType>[] = ["requiredNumber", "requiredString"]; // Working example, only matches to the names of the required properties


type BrokenExampleType = {
    requiredString: string;
    requiredNumber: number;
    optionalBoolean?: boolean;
    [keyword: string]: any; // <-- Only difference between this type and WorkingExampleType
}

let y: RequiredMembers<BrokenExampleType>[] = ["requiredNumber", "requiredString"]; // <-- Errors on these strings. "Type 'string' is not assignable to type 'undefined'. (2322)"

🙁 Actual behavior

RequiredMembers<BrokenExampleType> resolves to undefined.

🙂 Expected behavior

RequiredMembers<BrokenExampleType> resolves to "requiredString" | "requiredNumber"

[MartinJohns]

By design. Due to the indexer every string is a valid key, so keyof returning string is correct.

See #41966 for a workaround (omit the index signature).

[LoganGerber]

Due to the indexer every string is a valid key, so keyof returning string is correct.

I don't think that's where my problem was. It's in [K in keyof T]?: undefined extends T[K] ? never : K;, specifically the undefined extends T[K] ? never : K; always resolving to never. I know that keyof should always return string, but the issue is with the values, not the keys.

[LoganGerber]

Oh nevermind, I understand what you're saying now. It's the [keyof T] that's causing the issue when using it on BrokenExampleType, but it's working as intended. My bad, thanks for the help yall!