Clarify library limitations on 'Class modifiers reference'

[vladasp]

Page URL

https://dart.dev/language/modifier-reference.html

Page source

https://github.com/dart-lang/site-www/tree/main/src/language/modifier-reference.md

Describe the problem

Interface class are able to be extended.

The code below has no compilation errors

interface class Base {
  bool someMethod() {
    return Random().nextBool();
  }
}

class ExtendedBase extends Base {
  bool someMthNew() {
    return super.someMethod();
  }
}

class ImplementedBase implements Base {
  @override
  bool someMethod() {
    // TODO: implement someMth
    throw UnimplementedError();
  }
}

[parlough]

Thanks for opening an issue @vladasp!

As the documentation for the interface modifier specifies "Libraries outside of the interface’s own defining library can implement the interface, but not extend it." The code you included is all in one file, therefore it's all in one library and the interface modifier's limitations do not apply. If you moved ExtendedBase to another file, it should report a diagnostic.

The new class modifiers (base, interface, final, and sealed) all limit specific types of behavior outside of the current library, but not within it. The class modifiers documentation goes into detail.

Let us know if that helps. Thanks again for opening an issue!

\cc @MaryaBelanger Perhaps we should include a small note at the top of the reference that this is the case for some modifiers?

[vladasp]

Thanks for opening an issue @vladasp!

As the documentation for the interface modifier specifies "Libraries outside of the interface’s own defining library can implement the interface, but not extend it." The code you included is all in one file, therefore it's all in one library and the interface modifier's limitations do not apply. If you moved ExtendedBase to another file, it should report a diagnostic.

The new class modifiers (base, interface, final, and sealed) all limit specific types of behavior outside of the current library, but not within it. The class modifiers documentation goes into detail.

Let us know if that helps. Thanks again for opening an issue!

\cc @MaryaBelanger Perhaps we should include a small note at the top of the reference that this is the case for some modifiers?

@parlough Thank you for answer and currently it become clear for me.
But it still seems to me that the Reference page should be more precise in this regard. Since the table is provided on the separate page and there is no information that it is only valid outside of the library. In addition, the page structure:
Class modifiers:

• Overview and usages
• Class modifiers for API maintainers
• Reference

contains a separate item for API maintainers, which suggests that only this particular section describes behaviour outside the library.