Check for cyclic type variable bounds.

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 get currentElement;
   Set<Node> get superUses;
 
@@ skipping 2950 matching lines @@
       if (nameSet.contains(typeName)) {
         error(typeNode, MessageKind.DUPLICATE_TYPE_VARIABLE_NAME,
               {'typeVariableName': typeName});
       }
       nameSet.add(typeName);
 
       TypeVariableElement variableElement = typeVariable.element;
       if (typeNode.bound != null) {
         DartType boundType = typeResolver.resolveTypeAnnotation(
             typeNode.bound, scope, element, onFailure: warning);
-        if (boundType != null && boundType.element == variableElement) {
-          // TODO(johnniwinther): Check for more general cycles, like
-          // [: <A extends B, B extends C, C extends B> :].
-          warning(node, MessageKind.CYCLIC_TYPE_VARIABLE,
-                  {'typeVariableName': variableElement.name});
-        } else if (boundType != null) {
-          variableElement.bound = boundType;
-        } else {
-          // TODO(johnniwinther): Should be an erroneous type.
-          variableElement.bound = compiler.objectClass.computeType(compiler);
+        variableElement.bound = boundType;
+
+        void checkTypeVariableBound() {
+          Link<TypeVariableElement> seenTypeVariables =
+              const Link<TypeVariableElement>();
+          seenTypeVariables = seenTypeVariables.prepend(variableElement);
+          DartType bound = boundType;
+          while (bound.element.isTypeVariable()) {
+            TypeVariableElement element = bound.element;
+            if (seenTypeVariables.contains(element)) {
+              if (identical(element, variableElement)) {
+                // Only report an error on the checked type variable to avoid
+                // generating multiple errors for the same cyclicity.
+                warning(typeNode.name, MessageKind.CYCLIC_TYPE_VARIABLE,
+                    {'typeVariableName': variableElement.name});
+              }
+              break;
+            }
+            seenTypeVariables = seenTypeVariables.prepend(element);
+            bound = element.bound;
+          }
         }
+        compiler.enqueuer.resolution.addPostProcessAction(
+            element, checkTypeVariableBound);
       } else {
         variableElement.bound = compiler.objectClass.computeType(compiler);
       }
       nodeLink = nodeLink.tail;
       typeLink = typeLink.tail;
     }
     assert(typeLink.isEmpty);
   }
 }
 
@@ skipping 835 matching lines @@
     return e;
   }
 
   /// Assumed to be called by [resolveRedirectingFactory].
   Element visitReturn(Return node) {
     Node expression = node.expression;
     return finishConstructorReference(visit(expression),
                                       expression, expression);
   }
 }