I have the following isResult function:

export function isResult<
    R extends CustomResult<string, Record<string, any>[]>,
    K extends R[typeof _type]
>(result: R, type: K): result is Extract<R, { [_type]: K }> {
    return result[_type] === type;
}

const users = getUsers("abc");
if (isResult(users, "user")) {
    console.log(users.userProp);
} else {
    console.log(users.adminProp);
}

My goal is to

1. Allow auto suggestions of the second argument of isResult() (Here: “user” | “admin”)
2. Make isResult() narrow the types properly (it should not show any errors)

But I can only achieve either one. The first goal (auto complete) is achived by using K extends R[typeof _type]. And the second (narrowing properly) only works with K extends string.

It seems that Typescript interprets K as the literal “user” when using K extends string but still the entire union “user” | “admin” when using K extends R[typeof _type].

The only way I found that works with both goals is the use of K extends R[typeof _type] and isResult(users, "user" as const).

I have 2 Questions:

1. Why does it work without as const when using K extends string?
2. How can I avoid the use of as const while keeping the auto complete of the second argument of isResult()?