Type narrowing is lost when using logical OR assignment

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Issue 类型
缺陷
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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。

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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)

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