Incorrect inferance of never type (regression in 4.3.2)

[marijnh]

Bug Report

🔎 Search Terms

"4.3" never

🕗 Version & Regression Information

The code typechecks in 4.2.3, but no longer does so in 4.3.2.

⏯ Playground Link

Playground link with relevant code

💻 Code

export function test() {
  let found: null | string = null;
  ["hi"].forEach(s => found = s);
  if (!found) return;
  console.log(found!.length);
}

🙁 Actual behavior

The last line produces an error "Property 'length' does not exist on type 'never'". This goes away if the early return is removed. It seems that narrows the type of found from null to never, but that seems inappropriate in this situation (and hard to work around without any casts).

🙂 Expected behavior

I'd expect the exclamation point to be enough to make the code typecheck (as it did in previous versions).

[MartinJohns]

This seems intentional: #44025 (comment)

this goes away if the early return is removed. It seems that narrows the type of found from null to never, but that seems inappropriate in this situation (and hard to work around without any casts).

This is a known issue as well: #9998

Due to this the type is always known as null, and never as string. So when you return TypeScript knows it's not null, but it can only ever be null, so it must be never.

---

A simple workaround is to avoid the inferrence of found to be null by using a type assertion:

let found: null | string = null as null | string

[marijnh]

Seems to be a duplicate of #44025, indeed.