Disallow cyclic mixins.

sdk/lib/_internal/compiler/implementation/resolution/members.dart

 // Copyright (c) 2012, 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.
 
 part of resolution;
 
 abstract class TreeElements {
   Element operator[](Node node);
   Selector getSelector(Send send);
   DartType getType(Node node);
@@ skipping 491 matching lines @@
     int illegalFlags = modifiers.flags & ~Modifiers.FLAG_ABSTRACT;
     if (illegalFlags != 0) {
       Modifiers illegalModifiers = new Modifiers.withFlags(null, illegalFlags);
       CompilationError error =
           MessageKind.ILLEGAL_MIXIN_APPLICATION_MODIFIERS.error(
               [illegalModifiers]);
       compiler.reportMessage(compiler.spanFromSpannable(modifiers),
                              error, Diagnostic.ERROR);
     }
 
+    // In case of cyclic mixin applications, the mixin chain will have
+    // been cut. If so, we have already reported the error to the
+    // user so we just return from here.
+    ClassElement mixin = mixinApplication.mixin;
+    if (mixin == null) return;
+
     // Check that the mixed in class has Object as its superclass.
-    ClassElement mixin = mixinApplication.mixin;
     if (!mixin.superclass.isObject(compiler)) {
       CompilationError error = MessageKind.ILLEGAL_MIXIN_SUPERCLASS.error();
       compiler.reportMessage(compiler.spanFromElement(mixin),
                              error, Diagnostic.ERROR);
     }
 
     // Check that the mixed in class doesn't have any constructors.
     mixin.forEachLocalMember((Element member) {
       if (member.isGenerativeConstructor() && !member.isSynthesized) {
         CompilationError error = MessageKind.ILLEGAL_MIXIN_CONSTRUCTOR.error();
@@ skipping 2370 matching lines @@
     Link<DartType> interfaces = const Link<DartType>();
     interfaces = interfaces.prepend(mixinType);
     assert(mixinApplication.interfaces == null);
     mixinApplication.interfaces = interfaces;
 
     assert(mixinApplication.mixin == null);
     mixinApplication.mixin = mixinType.element;
     mixinApplication.mixin.ensureResolved(compiler);
     mixinApplication.addDefaultConstructorIfNeeded(compiler);
     calculateAllSupertypes(mixinApplication);
+
+    // Check for cycles in the mixin chain.
+    ClassElement previous = mixinApplication;
+    ClassElement current = mixinApplication.mixin;
+    while (current != null && current.isMixinApplication) {
+      MixinApplicationElement currentMixinApplication = current;
+      if (currentMixinApplication == mixinApplication) {
+        CompilationError error = MessageKind.ILLEGAL_MIXIN_CYCLE.error(
+            [current.name, previous.name]);
+        compiler.reportMessage(compiler.spanFromElement(mixinApplication),
+                               error, Diagnostic.ERROR);
+        // Set the mixin to null to avoid running into issues where
+        // traversing the members will result in infinite recursion.
+        mixinApplication.mixin = null;
+        break;
+      }
+      previous = current;
+      current = currentMixinApplication.mixin;
+    }

[Johnni Winther, 2013/01/22 13:16:41]

I don't think this handles the case when [mixinApplication] is not part of the
cyclic but on a cyclic. Like

typedef C = Object with B;
typedef A = Object with B;
typedef B = Object with A;

Checking C would pose a problem.

   }
 
 
   // TODO(johnniwinther): Remove when default class is no longer supported.
   DartType visitTypeAnnotation(TypeAnnotation node) {
     return visit(node.typeName);
   }
 
   // TODO(johnniwinther): Remove when default class is no longer supported.
   DartType visitIdentifier(Identifier node) {
@@ skipping 595 matching lines @@
     return e;
   }
 
   /// Assumed to be called by [resolveRedirectingFactory].
   Element visitReturn(Return node) {
     Node expression = node.expression;
     return finishConstructorReference(visit(expression),
                                       expression, expression);
   }
 }