Fix type argument finalization of some recursive generic types (fixes #29949).

runtime/vm/class_finalizer.cc

 // Copyright (c) 2013, 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/class_finalizer.h"
 
 #include "vm/flags.h"
 #include "vm/hash_table.h"
 #include "vm/heap.h"
 #include "vm/isolate.h"
@@ skipping 962 matching lines @@
                         ref_super_type_arg.ToCString());
             }
             CheckRecursiveType(cls, ref_super_type_arg, pending_types);
             pending_types->Add(ref_super_type_arg);
             const Class& super_cls =
                 Class::Handle(ref_super_type_arg.type_class());
             const TypeArguments& super_args =
                 TypeArguments::Handle(ref_super_type_arg.arguments());
             // Mark as finalized before finalizing to avoid cycles.
             ref_super_type_arg.SetIsFinalized();
-            // Since the instantiator is different, do not pass the current
-            // instantiation trail, but create a new one by passing NULL.
+            // Although the instantiator is different between cls and super_cls,
+            // we still need to pass the current instantiation trail as to avoid
+            // divergence. Finalizing the type arguments of super_cls may indeed
+            // recursively require instantiating the same type_refs already
+            // present in the trail (see issue #29949).
             FinalizeTypeArguments(
                 super_cls, super_args,
                 super_cls.NumTypeArguments() - super_cls.NumTypeParameters(),
-                bound_error, pending_types, NULL);
+                bound_error, pending_types, instantiation_trail);
             if (FLAG_trace_type_finalization) {
               THR_Print("Finalized instantiated TypeRef '%s': '%s'\n",
                         String::Handle(super_type_arg.Name()).ToCString(),
                         ref_super_type_arg.ToCString());
             }
           }
         }
       }
       arguments.SetTypeAt(i, super_type_arg);
     }
@@ skipping 2805 matching lines @@
   ProgramVisitor::VisitFunctions(&function_visitor);
 
   class ClearCodeClassVisitor : public ClassVisitor {
     void Visit(const Class& cls) { cls.DisableAllocationStub(); }
   };
   ClearCodeClassVisitor class_visitor;
   ProgramVisitor::VisitClasses(&class_visitor);
 }
 
 }  // namespace dart