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 2366 matching lines @@
     mixinApplication.supertype = supertype;
 
     // The class that is the result of a mixin application implements
     // the interface of the class that was mixed in.
     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.mixin = resolveMixinFor(mixinApplication, mixinType);
     mixinApplication.addDefaultConstructorIfNeeded(compiler);
     calculateAllSupertypes(mixinApplication);
   }
 
+  ClassElement resolveMixinFor(MixinApplicationElement mixinApplication,
+                               DartType mixinType) {
+    ClassElement mixin = mixinType.element;
+    mixin.ensureResolved(compiler);
+
+    // Check for cycles in the mixin chain.
+    ClassElement previous = mixinApplication;  // For better error messages.
+    ClassElement current = 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);
+        // We have found a cycle in the mixin chain. Return null as
+        // the mixin for this application to avoid getting into
+        // infinite recursion when traversing members.
+        return null;
+      }
+      previous = current;
+      current = currentMixinApplication.mixin;
+    }
+    return mixin;
+  }
 
   // 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) {
     Element element = scope.lookup(node.source);
     if (element == null) {
@@ skipping 593 matching lines @@
     return e;
   }
 
   /// Assumed to be called by [resolveRedirectingFactory].
   Element visitReturn(Return node) {
     Node expression = node.expression;
     return finishConstructorReference(visit(expression),
                                       expression, expression);
   }
 }