Fails to perform type narrowing on functions that return "never"

Bug Report

🔎 Search Terms

type narrowing never

🕗 Version & Regression Information

• v4.5.2
• v4.4.4
• v4.3.5
• v4.2.3
• v4.1.5
• v4.0.5
• v3.9,7
• This is the behavior in every version I tried, and I reviewed the FAQ for entries about this.

⏯ Playground Link

Playground link with relevant code

💻 Code

type ShouldNeverReturnFunction = () => never;

let shouldNeverReturn: ShouldNeverReturnFunction | null = null;

function foo(): never {
  if (shouldNeverReturn === null) {
    throw new Error('The "shouldNeverReturn" function was not initialized.');
  }

  shouldNeverReturn();
}

🙁 Actual behavior

The TypeScript compiler refuses to compile this code.

🙂 Expected behavior

I expect the TypeScript compiler to compile this code.

The error message says that the foo function has “a reachable end point”, but that is clearly false, so the TypeScript compiler seems to be drunk. 🍺🥴

[RyanCavanaugh]

I think it’s in principle possible to expand the checking to cover unions, but this seems like a fairly rare case. The old workaround of return shouldNeverReturn(); works well here. We can look into it more if it’s commonly encountered

[MartinJohns]

#32695 mentions:

the function name resolves to a function type with an asserts return type or an explicit never return type annotation.

shouldNeverReturn does not resolve to such a function, it only is later on narrowed to one. Unfortunately the PR does not mention unions, so it’s not clear whether this is intentional (my guess) or not.