Bug报告

🔎 搜索词

控制流分析，解构赋值

🕗 版本与回归信息

这个已经存在了，但我认为在4.6中引入这个功能时可能已经修复了。

⏯ Playground链接

带有相关代码的Playground链接

💻 代码

export const reduce = <S, T extends S>(self: Iterable<T>, operation: (acc: S, t: T) => S): S => {
  const iter = self[Symbol.iterator]();
  let { value, done }: IteratorResult<T, unknown> = iter.next();
  if (done) throw new Error("Empty iterable can't be reduced.");
  let acc: S = value; // Type 'unknown' is not assignable to type 'S'.
  while (!done) {
    acc = operation(acc, value);
    ({ value, done } = iter.next());
  }
  return acc;
};

请注意，如果你不向上转型 iter.next() ,你将不会得到错误，但这并不是出于好的原因：

let {value, done} = iter.next(); // type is IteratorResult<T, any>
if (done) throw new Error("Empty iterable can't be reduced.");
// value is any and can be assigned to anything

一个更小的仓库是：

export function foo(iter: Iterator<number, string>) {
  let { value, done } = iter.next();
  if (done) return;
  let acc: number = value; // Type 'string | number' is not assignable to type 'number'. Type 'string' is not assignable to type 'number'.
  return acc;
}

🙁 实际行为

上面最后一个例子中的 value 被推断为 number | string ,它应该是 number

🙂 预期行为

我希望有与没有解构相同的推断：

export const reduce2 = <S, T extends S>(self: Iterable<T>, operation: (acc: S, t: T) => S): S => {
  const iter = self[Symbol.iterator]();
  let result: IteratorResult<T, unknown> = iter.next();
  if (result.done) throw new Error("Empty iterable can't be reduced.");
  let acc: S = result.value; // result.value is T (extends S)
  while (!result.done) {
    acc = operation(acc, result.value);
    result = iter.next();
  }
  return acc;
};