Error creating super mixin on class with no default 0 arg ctor

[MichaelRFairhurst]

Repro:

class NoDefaultZeroArgCtor {                                                     
  NoDefaultZeroArgCtor(int x);                                                   
                                                                                 
  int foo() => 122;                                                              
}                                                                                
                                                                                 
mixin FooMixin on NoDefaultZeroArgCtor {                                         
  int foo() => super.foo();                                                      
}                                                                                
                                                                                 
class UsesFoo extends NoDefaultZeroArgCtor with FooMixin {                       
  UsesFoo(int x) : super(x);                                                     
}                                                                                
                                                                                 
void main() {                                                                    
  print(new UsesFoo(0).foo());                                                   
} 

via dart:

lib/mixin.dart:1: Warning: Interpreting this as package URI, 'package:test/mixin.dart'.
lib/mixin.dart:1: Error: The superclass, 'NoDefaultZeroArgCtor', has no unnamed constructor that takes no arguments.

via dartanalyzer:

Analyzing lib/mixin.dart...
No issues found!

The latter seems desirable to me, though I can only hazard a guess at if there's a good reason why the former is the required/correct behavior.

[stereotype441]

Classifying as "area-language" for now since we're not sure whether analyzer or front end behavior is correct. @leafpetersen, @eernstg, @lrhn, can you let us know which behavior is correct, so we can reassign the bug to analyzer or front end as appropriate?

[lrhn]

The analyzer is correct, and so is the code.

Mixin declarations are not class declarations, and their super-class constraints are not super-classes. Mixin declarations have no constructors, and they do not need to call a super-constructor, so should make no difference which constructors their super-constraint classes have.

I get the same error from both dart2js and the VM, so it's most likely a front-end issue.
(It may be that both back-ends have implemented the same error with the same error message, but it's less likely than it being a front-end error).

If I change the code to:

class X {}
class NoDefaultZeroArgCtor implements X {                                                     
  NoDefaultZeroArgCtor(int x);                                                   
...
}                                                                                
                                                                                 
mixin FooMixin on NoDefaultZeroArgCtor, X {    
...

then the program runs. It seems that the front-end is introducing an abstract interface in the case where there are more than one super-constraint, and that interface does have an unnamed nullary constructor, so their hack works. It should work in the one-constraint case too.

[stereotype441]

@kmillikin

[kmillikin]

The corresponding code with old-style super mixins would have had:

class FooMixin extends NoDefaultZeroArgCtor {                                         
  int foo() => super.foo();                                                      
}

which would have signaled the same error; and there was no way to repair it by adding a constructor to FooMixin because then it couldn't be used as a mixin.

So this is a problem affecting new code using mixin declarations, but not for porting existing code using old-style super mixins because it wouldn't have worked in the first place.

I'm testing a fix.

[kwalrath]

I'm running into this bug with the 2.1.0 candidate.

/cc @JekCharlsonYu @dgrove

[kwalrath]

See https://travis-ci.org/dart-lang/site-www/builds/455227237. I've got a smaller repro, if you want it.

[kwalrath]

Here's the smaller repro (passes analysis but fails to run unless you swap in the commented-out lines):

mixin Musical {
  bool canPlayPiano = false;
  bool canCompose = false;
  bool canConduct = false;

  void entertainMe() {
    if (canPlayPiano) {
      print('Playing piano');
    } else if (canConduct) {
      print('Waving hands');
    } else {
      print('Humming to self');
    }
  }
}

abstract class Performer {
  Performer(String name);
  String name;
}

class Musician extends Performer with Musical {
//  Musician.withName(String name) : super(name);
//  Musician() : super('Anonymous');
  Musician(String name) : super(name);
}


mixin MusicalPerformer on Musician {
  bool canDance = true;

  @override
  void entertainMe() =>
      canDance ? dance() : super.entertainMe();

  void dance() => print('Dancing');
}

class SingerDancer extends Musician with MusicalPerformer {
//  SingerDancer(String name) : super.withName(name);
  SingerDancer(String name) : super(name);
}


void main() {
//  var musician = Musician.withName('Kathy');
  var musician = Musician('Kathy'); // Musician.withName('Kathy');
  musician.canPlayPiano = true;
  musician.entertainMe(); // Playing piano

  var singerDancer = SingerDancer('Todd');
  singerDancer.entertainMe(); // Dancing
}

[kwalrath]

I suppose @MichaelRFairhurst's repro is better/shorter. :)

[kmillikin]

This is fixed but the fix somehow missed the final 2.1 dev release. The fix landed before the last push to dev, so I assumed that it had made it in and didn't request a merge to dev. I'm sorry about that.

So this is now an (extremely) annoying bug in the new 2.1 feature that will be fixed in the next release.

[athomas]

@kmillikin if this is supposed to go into the 2.1.1 stable release, please file a merge-to-stable request.

[lukepighetti]

Not sure if this is helpful, but here is an example that fails on 2.1.0

class Cook = Person with AwesomeCookingSkills;

class Person {
  final String name;

  Person(this.name);
}

mixin AwesomeCookingSkills on Person {
  String get bestDish => "$name makes the best chocolate.";
}

main() {
  final cook = Cook("Johnny Appleseed");
  /// Error: The superclass, 'Person', has no unnamed constructor that takes no arguments.
  
  print(cook.bestDish);
}

And here is a Gist if it's useful for tracking this https://dartpad.dartlang.org/b4f766621b01ffa5270e5de93ef592b5

[lukepighetti]

Did this miss 2.1.1 or is it just not in the changelog? https://github.com/dart-lang/sdk/blob/master/CHANGELOG.md#211---2019-02-18

[kmillikin]

It should be in 2.1.1.

[athomas]

ad2db25 is in 2.1.1:

$ git tag --contains ad2db25
2.1.1