generics - Typescript编译器从不错误：键入'string |

这是我得到的一段代码：

interface Obj {
  a: number
  b: string
}

const obj: Obj = {
  a: 1,
  b: 'hi'
}

function fn(key: keyof Obj, value: Obj[keyof Obj]) {
  let foo = obj[key]
  obj[key] = value
}

fn("a", 2)

所以我想做的是，我希望函数fn()能够更新对象obj属性，该函数的第一个参数是obj具有（在Obj接口中定义）的任何键，第二个参数是您想要为其提供的值键。

但是，obj[key] = value此行中出现错误的打字稿弹出窗口是：

Type 'string | number' is not assignable to type 'never'.
  Type 'string' is not assignable to type 'never'.(2322)

这是屏幕截图：

如果将鼠标悬停在变量上foo（图片中的第13行），它会在这里发生奇怪的事情：

let foo: string | number

which means, obj[key]'s type is string | number, but the error says obj[key] type is never.

so my first question is: How come a string | number type magically becomes a never type? Is there any way to fix this?

Then i got another piece of code which solves this problem:

interface Obj {
  a: number
  b: string
}

const obj: Obj = {
  a: 1,
  b: 'hi'
}

function fn<K extends keyof Obj>(key: K, value: Obj[K]) {
  obj[key] = value
}

fn("a", 2)

Therefore my second question would be: why using Generics solve the problem and what the hack is the keyword extends here?

BTW, all the code are tested in typescript 3.7.5 version.

I am not a native English speaker, hope i explained my confusion clearly.

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