Cannot use override keyword to override methods of Object without explicitly extending Object

[kmxz]

Bug Report

🔎 Search Terms

override, TS4112

🕗 Version & Regression Information

It happens in 4.4.2 and has been there since the introduction of override keyword (#39669).

⏯ Playground Link

https://www.typescriptlang.org/play?#code/MYGwhgzhAECC0G8BQ1oHsBuBTATjglgCZbQAuaAyqQQHYDmAFAJSKo5akCuON0A5ADM0aPgG5oAXyQSgA

💻 Code

class A {
  override toString() { return 'foo'; }
}

🙁 Actual behavior

TS4112: This member cannot have an 'override' modifier because its containing class 'A' does not extend another class.

🙂 Expected behavior

It should be allowed, since the method overrides Object.prototype.toString.

Per ECMAScript spec §15.7.12.7, a class without an explicit extends clause would be implicitly inheriting from Object. Therefore, such classes should be allowed to explicitly override methods declared in Object.

[kmxz]

FYI, I have a fix for this at kmxz@08ef62d. I haven't created a pull request as this issue is not yet labelled "help wanted", but I'm happy to create one at any time.

---

I do realize that there's another issue with overriding a method declared in Object:

// Error "TS2416: Property 'toString' in type 'A' is not assignable to the same property in base type 'Object'. Type '() => number' is not assignable to type '() => string'".
class A extends Object {
  override toString() { return 0; }
}

// No errors!
class B {
  override toString() { return 0; }
}

It makes sense to issue an error in the second case, since TypeScript already treats all classes as subclasses of Object when resolving property accesses. I'm just a bit concerned that it could be a breaking change for some users.