Finalizing an upper bound during a compile-time type test may lead to cycles.

runtime/vm/object.cc

 // 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.
 
 #include "vm/object.h"
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "vm/assembler.h"
 #include "vm/cpu.h"
@@ skipping 15666 matching lines @@
   if (IsTypeParameter()) {
     const TypeParameter& type_param = TypeParameter::Cast(*this);
     if (other.IsTypeParameter()) {
       const TypeParameter& other_type_param = TypeParameter::Cast(other);
       if (type_param.Equals(other_type_param)) {
         return true;
       }
     }
     const AbstractType& bound = AbstractType::Handle(type_param.bound());
     // We may be checking bounds at finalization time and can encounter
-    // a still unfinalized bound.
-    if (!bound.IsFinalized() && !bound.IsBeingFinalized()) {
-      ClassFinalizer::FinalizeType(
-          Class::Handle(type_param.parameterized_class()),
-          bound,
-          ClassFinalizer::kCanonicalize);
-      type_param.set_bound(bound);
+    // a still unfinalized bound. Finalizing the bound here may lead to cycles.
+    if (!bound.IsFinalized()) {
+      return false;    // TODO(regis): Return "maybe after instantiation".
     }
     if (bound.IsMoreSpecificThan(other, bound_error)) {
       return true;
     }
     return false;  // TODO(regis): We should return "maybe after instantiation".
   }
   if (other.IsTypeParameter()) {
     return false;  // TODO(regis): We should return "maybe after instantiation".
   }
   const Class& cls = Class::Handle(type_class());
@@ skipping 6259 matching lines @@
   return tag_label.ToCString();
 }
 
 
 void UserTag::PrintJSONImpl(JSONStream* stream, bool ref) const {
   Instance::PrintJSONImpl(stream, ref);
 }
 
 
 }  // namespace dart