Better message for `override` inside `extension type`

[FMorschel]

While testing a code for #59820 as bellow:

class C {
  C._(this.x);
  final int x;
}

extension type E(C c) implements C {
  @override
  int get x => 0;
}

On dartpad set to main, I get:

The getter doesn't override an inherited getter.

I don't think that is true considering implements C. Can anyone confirm this?

[eernstg]

An extension type can't override anything, it can only redeclare a member.

The difference is that when a member has been overridden, an invocation will always call the most specific implementation of that member, no matter whether that most specific implementation is known from the static type or not. An extension type member is only invoked when it is the statically known declaration for that member name. For example:

import 'package:meta/meta.dart';

class C {
  C._(this.x);
  final int x;
}

extension type E(C c) implements C {
  @redeclare
  int get x => 0;
}

The @redeclare metadata annotation indicates that there is a member of some superinterface (here: C) that also has the name x (and it is implied that the name clash was intended).

A redeclaration is much more flexible than an overriding declaration: The redeclaration can be a getter when the redeclared declaration is a method or vice versa (we can't do that in an overriding declaration), and the signatures don't have to be related.

The reason why there are no constraints is that the redeclaration is never invoked in a situation where it isn't known statically. In other words, if you're calling a redeclaration then a "jump to declaration" operation in an IDE will take you to the exact piece of code that will run at run time, there will never be a "more specific" redeclaration which will "override" it.

So you should simply use @redeclare rather than @override when the metadata sits on an extension type declaration.

[FMorschel]

Ohh, I see. Thanks for the explanation!

[FMorschel]

@eernstg would it be reasonable to reopen this as a request for a better message for @override inside extension types ? Maybe even mentioning @redeclare.

[eernstg]

Sounds like a good idea! @bwilkerson, WDYT?

[bwilkerson]

I agree, a better message would be helpful to users.