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

```ts
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.

