推断对象键值的特定类型

假设我有这个：

type KeyForValueType<T, ValueType> = { [K in keyof T]: T[K] extends ValueType ? K : never; }[keyof T];

declare function fn1<T, ValueType>(obj: T, key: KeyForValueType<T, ValueType>): void;

fn1 will work as expected, it will properly limit the key I can use based on the type of value of that key:

fn1<{ a: number; b: string }, string>({ a: 11, b: 'str' }, 'a'); // error as expected
fn1<{ a: number; b: string }, string>({ a: 11, b: 'str' }, 'b'); // works
fn1<{ a: number; b: string }, number>({ a: 11, b: 'str' }, 'a'); // works
fn1<{ a: number; b: string }, number>({ a: 11, b: 'str' }, 'b'); // error as expected

I got the above from here: https://stackoverflow.com/a/51476941/520229

但是，当实际实现该函数时，我无法使它识别值的类型：

// with generic ValueType
function fn2<T, ValueType>(obj: T, key: KeyForValueType<T, ValueType>): void {
  const value: ValueType = obj[key];
  console.log(value);
}

// or even fixed value type
function fn3<T>(obj: T, key: KeyForValueType<T, string>): void {
  const value: string = obj[key];
  console.log(value);
}

Check it out here.

我在这里做错什么了吗？这甚至可能与打字稿有关？