Typescript .根据枚举输入参数返回不同的对象类型

我有模板名称enum，对于每个枚举值，函数fetchTemplate必须返回它自己的响应类型.

enum KeyEnum {
  VK = 'VK',
  VIBER = 'VIBER',
}

type VkResponse = { vk: string };
type ViberResponse = { viber: number };

type ReturnTypes = {
  [KeyEnum.VK]: VkResponse;
  [KeyEnum.VIBER]: ViberResponse;
};

我用这个签名创建了函数fetchTemplate:

const fetchTemplate = <T extends KeyEnum>(key: T): ReturnTypes[T]

在这具身体上:

const fetchTemplate = <T extends KeyEnum>(key: T): ReturnTypes[T] => {
  if (key === KeyEnum.VK) {
    return { vk: 'someString' }; <<- Here i got TS2322 error (description below)
  }

  if (key === KeyEnum.VIBER) {
    return { viber: 1 } as ReturnTypes[T]; <<- No problems, but i dont want use "as"
  }
};

# Unnecessary type definition in the left, just for testing
const vkRes: VkResponse = fetchTemplate(KeyEnum.VK);
const viberRes: ViberResponse = fetchTemplate(KeyEnum.VIBER);

但在函数体中，我不能只返回{vk: 'someString'}或{viber:1}，因为我得到了TS2322:

TS2322:
Type  { vk: string; }  is not assignable to type ReturnTypes[T]
Type  { vk: string; }  is not assignable to type  VkResponse & ViberResponse
Property  viber  is missing in type  { vk: string; }  but required in type ViberResponse 

为什么在这种情况下，TypeScrip不能进行文字缩小？为什么我需要使用"as ReturnTypes[T]"，如何避免？如何正确地将key类型从T缩小到特定的.VK或.VIBER，并在IFS块中显式定义某些(不是联合的)返回类型？

TypeScript v5.2.2
Node v20.8.0

这个也不起作用:

  switch (key) {
    case KeyEnum.VIBER:
      return { viber: 1 }; <<- The same TS2322 error
  }