打字稿中的一般约束是什么？

Question

extract.ts

function extract1<S>(source: S, key: keyof S) {
  return source[key]
}
function extract2<S, K extends keyof S>(source: S, key: K) {
  return source[key]
}

main.ts

const source = {foo: 1, bar: '2'}

// Type is deduced as `string | number`
const result1 = extract1(source, 'foo')

// Type is deduced as `number`
const result2 = extract2(source, 'foo')

extract1和extract2之间有什么区别，为什么可以精确推断出extract2的返回类型，而不能正确推断出extract1？