Typescript 联合中的分布条件类型排序影响

类型

type Diff<T, U> = T extends U ? never : T; 

---

这就像数学中的集差；它给出了在第二种类型中找不到的第一种类型的值。

//从T中删除可分配给U的类型。您希望从T30和T31中得到什么类型？糟糕，我昨晚看到了逗号。

// f last position
type T30 = Diff<
  "a" | "b" | "c" | "d",
  "a" | "c" | "f"
>; //type is:  "b" | "d"

// f first position
type T31 = Diff<
  "f" | "a" | "b" | "c" | "d",
  "a" | "c"
>; //type is: "b" | "d" | "f"
}