Move Null type to the Bottom in the VM (fixes #28025).

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/become.h"
@@ skipping 3723 matching lines @@
   Zone* zone = Thread::Current()->zone();
   Class& thsi = Class::Handle(zone, cls.raw());
   while (true) {
     ASSERT(!thsi.IsVoidClass());
     // Check for DynamicType.
     // Each occurrence of DynamicType in type T is interpreted as the dynamic
     // type, a supertype of all types.
     if (other.IsDynamicClass()) {
       return true;
     }
+    // Check for NullType, which, as of Dart 1.5, is a subtype of (and is more
+    // specific than) any type. Note that the null instance is not handled here.
+    if (thsi.IsNullClass()) {
+      return true;
+    }
     // In the case of a subtype test, each occurrence of DynamicType in type S
     // is interpreted as the bottom type, a subtype of all types.
     // However, DynamicType is not more specific than any type.
     if (thsi.IsDynamicClass()) {
       return test_kind == Class::kIsSubtypeOf;
     }
-    // Check for NullType, which is only a subtype of ObjectType, of
-    // DynamicType, or of itself, and which is more specific than any type.
-    if (thsi.IsNullClass()) {
-      // We already checked for other.IsDynamicClass() above.
-      return (test_kind == Class::kIsMoreSpecificThan) ||
-             other.IsObjectClass() || other.IsNullClass();
-    }
     // Check for ObjectType. Any type that is not NullType or DynamicType
     // (already checked above), is more specific than ObjectType.
     if (other.IsObjectClass()) {
       return true;
     }
     // Check for reflexivity.
     if (thsi.raw() == other.raw()) {
       const intptr_t num_type_params = thsi.NumTypeParameters();
       if (num_type_params == 0) {
         return true;
@@ skipping 12081 matching lines @@
       if (call.IsSubtypeOf(type_arguments, other_signature,
                            other_type_arguments, bound_error, Heap::kOld)) {
         return true;
       }
     }
   }
   if (!instantiated_other.IsType()) {
     return false;
   }
   other_class = instantiated_other.type_class();
+  if (IsNull()) {
+    ASSERT(cls.IsNullClass());
+    // As of Dart 1.5, the null instance and Null type are handled differently.
+    // We already checked for other.IsDynamicType().
+    return other_class.IsNullClass() || other_class.IsObjectClass();
+  }
   return cls.IsSubtypeOf(type_arguments, other_class, other_type_arguments,
                          bound_error, NULL, Heap::kOld);
 }
 
 
 bool Instance::OperatorEquals(const Instance& other) const {
   // TODO(koda): Optimize for all builtin classes and all classes
   // that do not override operator==.
   return DartLibraryCalls::Equals(*this, other) == Object::bool_true().raw();
 }
@@ skipping 679 matching lines @@
     }
     return false;
   }
   if (other.IsMalbounded()) {
     ASSERT(Isolate::Current()->type_checks());
     if ((bound_error != NULL) && bound_error->IsNull()) {
       *bound_error = other.error();
     }
     return false;
   }
-  if (other.IsObjectType() || other.IsDynamicType()) {
+  // Any type is a subtype of (and is more specific than) Object and dynamic.
+  // As of Dart 1.5, the Null type is a subtype of (and is more specific than)
+  // any type.
+  if (other.IsObjectType() || other.IsDynamicType() || IsNullType()) {
     return true;
   }
   Zone* zone = Thread::Current()->zone();
   if (IsBoundedType() || other.IsBoundedType()) {
     if (Equals(other)) {
       return true;
     }
     // Redundant check if other type is equal to the upper bound of this type.
     if (IsBoundedType() &&
         AbstractType::Handle(BoundedType::Cast(*this).bound()).Equals(other)) {
@@ skipping 6407 matching lines @@
   return UserTag::null();
 }
 
 
 const char* UserTag::ToCString() const {
   const String& tag_label = String::Handle(label());
   return tag_label.ToCString();
 }
 
 }  // namespace dart