Type narrowing is lost when using logical OR assignment

未关闭
#44,553 2 条评论 1 个 reaction 已指派 0 人 在 GitHub 查看

还没有人认领这个 Issue。

评估

难度
4/5
预计耗时
3-5 天
新手友好度
35/100
Issue 类型
缺陷
描述清晰度
描述清楚
活跃度
停滞
技术栈
typescript
领域
compilers

调研方向

从链接的 TypeScript Playground 开始，在所述版本中重现 curr || (curr = {}) 与 curr ||= {} 之间的差异。跟踪逻辑赋值的控制流缩窄行为；当等效代码能够通过类型检查且不再出现报告的 TS2322 错误时，即视为完成。

由索引模型根据 Issue 内容生成。

描述

Bug Domain: check: Control Flow

Bug Report

🔎 Search Terms

logical assignment, narrowing

🕗 Version & Regression Information

The behaviour is consistent across all versions since the introduction of logical assignment operators in TypeScript 4.0.
Checked range from 4.0.5 to Nightly, no changes in behaviour observed.

⏯ Playground Link

Playground link with relevant code

💻 Code
type F = () => number;

type R = {
  [ x: string ]: R | F | undefined
}

const isFunc = <T extends (...args: any[]) => any>(maybe:unknown) : maybe is T => typeof maybe === "function";

const check = (i: R) => {

  let tmp:R = i;

  let curr = tmp["something"]; // R | F | undefined, expected

  if( isFunc<F>(curr) ) return;

  curr // R | undefined, expected

  tmp = curr || (curr = {}); //ok, expected

  tmp = curr ||= {}; // Type 'R | F' is not assignable to type 'R'.
};
🙁 Actual behavior

Error 2322 on the second tmp assignment because R | F is not narrowed to R as with the curr || (curr = {})

🙂 Expected behavior

No error since both assignments to tmp are functionally equivalent
(I might be missing something obvious about how logical assignment is intended to work from the type system standpoint)

主要语言
Go
星标
111k
派生
14.4k
平均合并
1 天 19 小时
30 天内合并 PR
117

贡献指南

打开贡献指南

从这里开始

  1. 先读完整个 Issue，再读项目的贡献指南。
  2. 在 Issue 下留言说明你要接手 —— 这能避免两个人做同样的事。
  3. Fork 仓库，在一个分支上完成修改。
  4. 提交 Pull Request，并在描述里引用这个 Issue 编号。

microsoft/TypeScript 的其他 Issue

查看 microsoft/TypeScript 的全部 Issue

相似的 Issue

更多 Go Issue

把新 issue 发到你的邮箱

精选适合新手参与的 GitHub issue 摘要。