instanceof narrowing should respect type parameter bounds for covariant types

[ethanresnick]

🔎 Search Terms

instanceof typeguard type parameter bounds

🕗 Version & Regression Information

This is the behavior in every version I tried, and I reviewed the FAQ for entries about instanceof

⏯ Playground Link

https://www.typescriptlang.org/play/?target=99#code/KYDwDg9gTgLgBAYwDYEMDOa4GECuaYQC2AolFNADxYQAmwcoMwAdjZgEQCC7cAPnOwBC7AHwMQTVplLkocAN4AoOHCjAUNCMyQBPRLWAAubAYDci5fub4oOBASgAKCGBgBLLWmPz9dY9ToAGjhCYAwUAHMjOBs3Zgi4AF8ASgVLFTQcMGAnF3dPADpQ8Kjk8xUVGAALNzQChAM4AF44PI9rerNLRMUexTpkFDUrfDgQYxxmAGtmCAB3ZnNFNwAzRxA4OPwUZgRgCBXsPAISMmhUpRUQTrpFAHo7lQA9AH5exSA

💻 Code

export class CustomError<Code extends "A" | "B"> extends Error {
  readonly code: Code;

  constructor(options: { code: Code, message: string }) {
    super(options.message);
    this.code = options.code;
  }
}

declare const x: unknown;

if(x instanceof CustomError) {
  x.code // inferred as any
}

🙁 Actual behavior

x.code is any, because x is only narrowed to CustomError<any>.

🙂 Expected behavior

It would be much more useful to narrow Code in CustomError to its bounds, rather than to any. So x would become CustomError<"a" | "b"> and then x.code could usefully be "A" | "B"

Additional information about the issue

No response