Bug Report

I found a curious situation in Typescript 4.8 where a variable of type unknown narrowed down to {} behaves differently than a variable of type {} from the beginning.

🔎 Search Terms

4.8, narrow, in
Not sure if it is related to #50527.

🕗 Version & Regression Information

• I was unable to test this on prior versions because it relies on a new 4.8 feature.

⏯ Playground Link

Playground link with relevant code

💻 Code

const f = (x: {}, y: unknown) => {
  if (!("a" in x)) {
    return;
  }
  console.log(x);
  // x stays {} (is not narrowed), not optimal but at least not wrong

  if (!y) {
    return;
  }
  // y is narrowed to {}

  if (!("a" in y)) {
    return;
  }
  console.log(y);
  // y is narrowed to never, which is clearly incorrect, and pretty strange because
  // it had the same type as x before and we run the same guard
}

🙁 Actual behavior

Narrowing unknown to {} and then using the in operator results in an incorrect type, whereas it doesn’t happen when starting with a variable of type {}.

🙂 Expected behavior

Narrowing should behave the same way whether we start with the type {} or narrow unknown down to {}.