Can't write a 'pick' function that accurately infers type arguments

declare function pick<T, K extends keyof T>(obj: T, propNames: K[]): Pick<T, K>;

let x = pick({ a: 10, b: 20, c: 30 }, ["a", "c"]);
let b = x.b;

Expected: Error on accessing x.b.
Actual: No error.

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