将字符串枚举类型转换为联合类型的枚举值

Question

如何获得一个打字稿字符串枚举的联合类型？

enum MyEnum {
  A = 'a', // Values are different from keys, so keyof will not help here.
  B = 'b',
}

给定类似上面的枚举类型，如何获得联合类型“a”| “b”？