private Type merge(Type c1, Type c2) {
            ClassType class1 = (ClassType) c1;
            List<Type> act1 = class1.getTypeArguments();
            ClassType class2 = (ClassType) c2;
            List<Type> act2 = class2.getTypeArguments();
            ListBuffer<Type> merged = new ListBuffer<Type>();
            List<Type> typarams = class1.tsym.type.getTypeArguments();

            while (act1.nonEmpty() && act2.nonEmpty() && typarams.nonEmpty()) {
                if (containsType(act1.head, act2.head)) {
                    merged.append(act1.head);
                } else if (containsType(act2.head, act1.head)) {
                    merged.append(act2.head);
                } else {
                    TypePair pair = new TypePair(c1, c2);
                    Type m;
                    if (mergeCache.add(pair)) {
                        m = new WildcardType(lub(upperBound(act1.head),
                                                 upperBound(act2.head)),
                                             BoundKind.EXTENDS,
                                             syms.boundClass);
                        mergeCache.remove(pair);
                    } else {
                        m = new WildcardType(syms.objectType,
                                             BoundKind.UNBOUND,
                                             syms.boundClass);
                    }
                    merged.append(m.withTypeVar(typarams.head));
                }
                act1 = act1.tail;
                act2 = act2.tail;
                typarams = typarams.tail;
            }
            assert(act1.isEmpty() && act2.isEmpty() && typarams.isEmpty());
            return new ClassType(class1.getEnclosingType(), merged.toList(), class1.tsym);
        }