Type narrowing is lost when using logical OR assignment

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)

Inizia con il TypeScript Playground collegato e riproduci la differenza tra curr || (curr = {}) e curr ||= {} nelle versioni indicate. Traccia il comportamento del restringimento del flusso di controllo per l'assegnazione logica; il lavoro è completato quando il codice equivalente supera il controllo dei tipi senza l'errore TS2322 segnalato.