Disallow cyclic mixins.

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

Index: sdk/lib/_internal/compiler/implementation/resolution/members.dart
diff --git a/sdk/lib/_internal/compiler/implementation/resolution/members.dart b/sdk/lib/_internal/compiler/implementation/resolution/members.dart
index 5b54c63c095337507ff4cfdb7fbafa67bb3885a9..5ab78047645498b9750af33bfb5f2914ec682b79 100644
--- a/sdk/lib/_internal/compiler/implementation/resolution/members.dart
+++ b/sdk/lib/_internal/compiler/implementation/resolution/members.dart
@@ -509,8 +509,13 @@ class ResolverTask extends CompilerTask {
                              error, Diagnostic.ERROR);
     }
 
-    // Check that the mixed in class has Object as its superclass.
+    // 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.
     if (!mixin.superclass.isObject(compiler)) {
       CompilationError error = MessageKind.ILLEGAL_MIXIN_SUPERCLASS.error();
       compiler.reportMessage(compiler.spanFromElement(mixin),
@@ -2901,6 +2906,25 @@ class ClassResolverVisitor extends TypeDefinitionVisitor {
     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.

   }