求助 typescript 类型声明

下面的类型声明有误

type Storable = undefined | boolean | number | string | Storable[] | {
  [key in string]: Storable;
}

报错: Index signature is missing in type "XXX" 有好方法能实现这种类型吗？

// 需要满足
let a: Storable

a = 5

a = "string"

a = [5, "string"]

a = {
  key: {
    whatever: [
      5,
      "string",
      {
        key: 23
      }
    ]
  }
}

// 但不能是 Function 或 Map 或其他千奇百怪的类型

// wrong
a = function () {
  // pass
}

// wrong
a = new Map()

// wrong
a = {
  key: new Map()
}