DFE fails to resolve methods inherited from mixin

[aam]

In Flutter code super.attach(owner); in RenderObjectWithChildMixin that is used as a mixin for RenderView should be resolved to RenderObject#attach but it gets unresolved:

file:///flutter/flutter/packages/flutter/lib/src/rendering/object.dart:2741:5: Warning: Method not found: 'super.attach'.
    super.attach(owner);
    ^

[aam]

Here is a snippet that demonstrates the problem:

class Base {
  void attach(String s) {
    print("Base attach $s");
  }
}

class ViewBase {
  void attach(String s) {
    print("Viewbase attach $s");
  }
}

abstract class Mixin implements Base {
  void attach(String s) {
    super.attach(s);
    print("Mixin attach $s");
  }
}

class View extends ViewBase with Mixin {
}

void main() {
  new View().attach("me");
}

Dart VM is happy to parse and run it:

$ ~/dart/dart-sdk/sdk/xcodebuild/DebugX64/dart mixin.dart 
Viewbase attach me
Mixin attach me
$ 

Both dartanalyzer and dart2js complain about Mixin referencing super

$ ~/dart/dart-sdk/sdk/xcodebuild/DebugX64/dart-sdk/bin/dartanalyzer mixin.dart 
Analyzing mixin.dart...
  error • The class 'Mixin' can't be used as a mixin because it references 'super' at mixin.dart:20:34 • mixin_references_super
1 error found.
$ ~/dart/dart-sdk/sdk/xcodebuild/DebugX64/dart-sdk/bin/dart2js mixin.dart 
mixin.dart:15:5:
Warning: Can't resolve 'attach' in a superclass of 'Mixin'.
    super.attach(s);
    ^^^^^^^^^^^^^^^
mixin.dart:20:34:
Error: Cannot use class 'Mixin' as a mixin because it uses 'super'.
class View extends ViewBase with Mixin {
                                 ^^^^^
mixin.dart:15:5:
Info: Use of 'super' in class used as mixin.
    super.attach(s);
    ^^^^^^^^^^^^^^^
Error: Compilation failed.
$ 

[a-siva]

@floitschG

[aam]

dart2js and dartanalyzer errors perhaps should be disregarded as they seem to be out of date with DEP https://github.com/gbracha/lessRestrictedMixins.

Per that DEP, DFE seems to be actually correct with it's diagnostic that super.attach() method is not found.

[floitschG]

I believe that the proposal requires an "extends" clause and not just "implements".

dart2js hasn't implemented this DEP.
I'm not sure about the analyzer, but I'm guessing it doesn't support it either.

Edit: since Peter assigned this issue to 'area-fasta', I'm guessing this might be related to the transition of the analyzer to the new frontend.

[sigmundch]

correct that dart2js doesn't implement it, analyzer has it under the --supermixin flag.

[aam]

@Hixie

[aam]

Ian provided good feedback that using 'extends' on mixin declaration leaves possibility to extend mixin directly outside of mixin application. On the other hand, if you use 'implements', then attempt to extend mixin will result in analyze-time error. See http://shortn/_5rg2Fqu4Qi for the example.

[peter-ahe-google]

From an implementation perspective, it would be extremely helpful if the work around of adding the superclass as specified could be used for now until the the language team has had a chance to hear feedback.

[aam]

Sure, Peter, I agree!

[lrhn]

The DEP has been implemented, so it's superseded by the ECMA language specification (4th Edition, Dec 2015), which contains the "new" mixin/super specification.

It's correct what Florian says, to have a super.m() invocation, your superclass'es interface must have an m() method.
When you apply a mixin derived from a class, A, with superclass B, the application must have a class that implements B. This (almost) ensures that the super.m() call has something to find (but it is possible to have an abstract superclass that doesn't have a concrete m method).

So

class B { int foo() => 37; }
class M extends B { int bar() => super.foo(); }
class A implements B { int foo() => 42; }
class N = A with M;

This works, A with M is allowed because A implements B, the superclass of M.

For the example above, if ViewBase implemented Base and Mixin extended Base, then it should be valid.

[peter-ahe-google]

@lrhn in your example, M has no superclass that provides a foo method.

[lrhn]

Thanks, fixed.

[peter-ahe-google]

Phew :-)