override isn't required in ambient contexts with noImplicitOverride

[Andarist]

🔎 Search Terms

override ambient type declaration class dts noImplicitOverride

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

https://www.typescriptlang.org/play/?noImplicitOverride=true&ts=5.8.0-dev.20241128&ssl=15&ssc=2&pln=1&pc=1#code/PTAEAEDsHsEkFsAOAbAlgY1QFwPIDcBTAJyNQBMCAuULIgVwIChGL1kBDIg0N9gZz6gAQv24BvRqFAAjapDrxpxANyMAvs1YcuPDgNAARYqkJlQBAB5YCkMoJF9xkmXIVKiqjSwK8dvfQ4EAEygYl5anNz+gkakpiGW1rb2oiESUtCEJOTcsqDyiirqjEA

💻 Code

// @noImplicitOverride: true

declare class Base {
  b: number;
}

declare class Derived extends Base {
  b: number; // no error
}

declare class Base2 {}

declare class Derived2 extends Base2 {
  override b: number; // error
}

🙁 Actual behavior

A property in a derived class in ambient context doesn't have to have override modifier with noImplicitOverride. override is still checked when present though.

🙂 Expected behavior

It feels slightly inconsistent. I couldn't find any note about this in the docs and I had to look into the source code and dig up the PR that implemented override to check if it's a bug or not.

Additional information about the issue

I believe this works like that since noImplicitOverride was introduced in #39669 in 4.3: TS playground

[whzx5byb]

Seems like there was a decision:

- Should this apply to declaration files?
  - Argument for yes: correctness for the same reason
  - Argument for no: convenience, no need to mess with declaration emitter
  - If no: Declaration emitter will need to emit `override` modifier (same way `any` works) if one was not present explicitly
  - Decision: Not enforced

Originally posted by @RyanCavanaugh in #9034