Compiler ignores 'instanceof' type guard after throwing an error

[Hanaasagi]

Bug Report

🔎 Search Terms

throw Error, never, instanceof, TS2339, Unreachable

🕗 Version & Regression Information

TypeScript Version 5.0.2

⏯ Playground Link

Playground link with relevant code

💻 Code

I am using instanceof in a piece of code to execute different operations based on the type of a variable. The code can be compiled successfully after commenting out the unimplemented function call. However, if I use unimplemented to throw an exception, the type information is missing . In this case, the compiler throws TS2339.

interface IValue {}

class Integer implements IValue {
  val: number;

  constructor(val: number) {
    this.val = val;
  }
}

function unimplemented(msg: string): never {
  throw new Error("It's unimplemented! " + msg);
}

function dispatch(x: IValue): number {
  unimplemented('function dispatch');

  if (x instanceof Integer) {
    return x.val;
  }

  return 0;
}

🙁 Actual behavior

Property 'val' does not exist on type 'IValue'. is reported.

🙂 Expected behavior

The code can be compiled successfully even if there is unreachable code, and the type check of instanceof should not be ignored.

[fatcerberus]

Duplicate of #26914.