tuple-union-switch.ts

type Item = StringItem | NumberIntem;
type StringItem = { key: "string"; value: string };
type NumberIntem = { key: "number"; value: number };

type ObjectUnionToEntryUnion<T extends Item> = T extends unknown
  ? [T["key"], T["value"]]
  : never;
type Entry = ObjectUnionToEntryUnion<Item>;

const items: Item[] = [
  { key: "number", value: 1 },
  { key: "string", value: "hey" },
];

// ERROR
{
  const entries = items.map<Entry>((item) => {
    return [item.key, item.value];
  });
}

// OK
// `key`の種類が増えたらその分`case`を書く羽目になる？
{
  const entries = items.map<Entry>((item) => {
    switch (item.key) {
      case "string":
        return [item.key, item.value];
      case "number":
        return [item.key, item.value];
      default:
        return item satisfies never;
    }
  });
}