typescript - 键在应该用于索引类型时却不能使用

Question

我有以下 TS 代码

type Fruit = { kind: "apple" } | { kind: "grape"; color: "green" | "black" };
type FruitTaste<TFruit extends Fruit> = TFruit["kind"] extends "apple"
  ? "good"
  : TFruit["color"] extends "green"
  ? "good"
  : "bad";

Playground link

TFruit["color"] 出现错误

Type '"color"' cannot be used to index type 'TFruit'.

但不应该，因为我们处于三元方面，其中 TFruit 应该仅限于 { kind: "grape";颜色:“绿色” | “black”}，并且 color 键应该存在。

奇怪的是，TS 的“运行时”版本不一定有任何问题:

type Fruit = { kind: "apple" } | { kind: "grape"; color: "green" | "black" };
const fruitTaste = (fruit: Fruit) =>
  fruit.kind === "apple" ? "good" : fruit.color === "green" ? "good" : "bad";

Playground link

这是为什么呢？如何实现 FruitTaste 类型？

Answer 1

我认为你可以通过分离类型 { kind: "apple"} 和 { kind: "grape"; 来实现它颜色:“绿色”| “黑色”}。

type Apple = { kind: "apple" };
type Grape = { kind: "grape"; color: "green" | "black" };
type Fruit = Apple | Grape;
type FruitTaste<TFruit extends Fruit> = TFruit extends Grape
    ? TFruit["color"] extends "green"
        ? "good"
        : "bad"
    : "good";