Fix ClassElement.allSupertypes to include interfaces of mixins (issue 29767)

pkg/analyzer/lib/src/dart/element/element.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library analyzer.src.dart.element.element;
 
 import 'dart:collection';
 import 'dart:math' show min;
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 1136 matching lines @@
       ClassElement currentElement = currentType.element;
       if (!visitedClasses.contains(currentElement)) {
         visitedClasses.add(currentElement);
         if (!identical(currentType, this.type)) {
           supertypes.add(currentType);
         }
         InterfaceType supertype = currentType.superclass;
         if (supertype != null) {
           typesToVisit.add(supertype);
         }
-        for (InterfaceType type in currentElement.interfaces) {
+        for (InterfaceType type in currentType.interfaces) {

[Jennifer Messerly, 2017/08/17 21:57:29]

oh wow, nice catch!

           typesToVisit.add(type);
         }
-        for (InterfaceType type in currentElement.mixins) {
+        for (InterfaceType type in currentType.mixins) {
           ClassElement element = type.element;
           if (!visitedClasses.contains(element)) {
             supertypes.add(type);
+            for (InterfaceType type in type.interfaces) {

[Jennifer Messerly, 2017/08/17 21:57:29]

there's also the weird case where a mixin's mixin is considered part of its
superclass:


class M1 { m() => print('M1.m()'); }

// this only works when M2 is declared with ClassTypeAlias syntax,
// and it mixes in exactly one other mixin
class M2 = Object with M1;

class B { m() => print('B.m()'); }
class C extends B with M2 {} // M2 is a legal mixin, strangely

main() {
  new C().m(); // prints M1.m
}

[Brian Wilkerson, 2017/08/17 22:49:25]

Is it still the case that classes used as mixins can only extend Object? If not,
does the superclass of the mixin need to be included in this list?

If the answer to either is "yes", then I think the for loop on line 1160 can be
replaced by

for (InterfaceType type in currentType.mixins) {
  typesToVisit.add(type);
}

[Jennifer Messerly, 2017/08/17 23:14:39]

oh yeah, good call! yeah with super mixins, the superclass of the mixin would be
considered as part of the `all supertypes` as well

+              typesToVisit.add(type);
+            }
           }
         }
       }
     }
   }
 
   /**
    * Compute a list of constructors for this class, which is a mixin
    * application.  If specified, [visitedClasses] is a list of the other mixin
    * application classes which have been visited on the way to reaching this
@@ skipping 8765 matching lines @@
 
   @override
   DartObject computeConstantValue() => null;
 
   @override
   void visitChildren(ElementVisitor visitor) {
     super.visitChildren(visitor);
     _initializer?.accept(visitor);
   }
 }