Confusing "isn't a valid override" error

[esDotDev]

With code like this:

abstract class AbstractMixin {
  R? watch<T extends ValueListenable<R>, R>({
    T? target,
    String? instanceName,
  });
}

mixin Foo implements AbstractMixin {
  R? watch<T extends ValueListenable<R>, R>({
    T? target,
    String? instanceName,
  }) {
    return target?.value; // simplified implementation, 
  }
}

Gives this error:

error: 'Foo.watch' ('R Function<T extends ValueListenable<R>, R>({String? instanceName, T? target})') 
isn't a valid override of 'AbstractMixin.watch' ('R Function<T extends ValueListenable<R>, R>({String? instanceName, T? target})').

Not sure what the problem is?

[lrhn]

That is a weird error. Appears to be analyzer only.

Reproduces in DartPad if you add:

abstract class ValueListenable<T>{
  T get value;
}
void main(){}

[eernstg]

Funny, the CFE had a similar issue a while ago, #47510 (also a case where we have 'S1 is not a correct override of S2' where the signatures S1 and S2 are identical). But the CFE doesn't fail on this one, and the analyzer doesn't fail on that old example. So, fun! 😁

[esDotDev]

Ya really weird, I have actually 6 methods in this class, 5 are fine, watch is the simplest one, and throws the error

[scheglov]

A smaller example for the analyzer is

abstract class A {
  void foo<T extends R, R>();
}

class B implements A {
  void foo<T extends R, R>() {}
}

[scheglov]

https://dart-review.googlesource.com/c/sdk/+/235167