Fix type checking of void type.

runtime/vm/flow_graph_builder.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/flow_graph_builder.h"
 
 #include "vm/ast_printer.h"
 #include "vm/bit_vector.h"
 #include "vm/code_descriptors.h"
 #include "vm/dart_entry.h"
@@ skipping 322 matching lines @@
 // specific than the given dst_type.
 static bool IsStaticTypeMoreSpecific(Value* value,
                                      const AbstractType& dst_type) {
   ASSERT(!dst_type.IsMalformed());
 
   // Any type is more specific than the Dynamic type and than the Object type.
   if (dst_type.IsDynamicType() || dst_type.IsObjectType()) {
     return true;
   }
 
-  // It is a compile-time error to explicitly return a value (including null)
-  // from a void function. However, functions that do not explicitly return a
-  // value, implicitly return null. This includes void functions. Therefore, we
-  // skip the type test here and trust the parser to only return null in void
-  // function.
-  if (dst_type.IsVoidType()) {
-    // TODO(regis): Should we perform this null test at run-time?
-    return true;
-  }
-
   // Do not perform type check elimination if this optimization is turned off.
   if (!FLAG_eliminate_type_checks) {
     return false;
   }
 
   // If nothing is known about the value, as is the case for passed-in
   // parameters, and since dst_type is not one of the tested cases above, then
   // the type test cannot be eliminated.
   if (value == NULL) {
     return false;
   }
 
+  // If the value is the null constant, its type (NullType) is more specific
+  // than the destination type, even if the destination type is the void type,
+  // since a void function is allowed to return null.
+  if (value->IsConstant() && value->AsConstant()->value().IsNull()) {
+    return true;
+  }
+
+  // Functions that do not explicitly return a value, implicitly return null,
+  // except generative constructors, which return the object being constructed.
+  // It is therefore acceptable for void functions to return null.
+  // In case of a null constant, we have already returned true above, else we
+  // return false here.
+  if (dst_type.IsVoidType()) {
+    return false;
+  }
+
   // Consider the static type of the value.
   const AbstractType& static_type = AbstractType::Handle(value->StaticType());
   ASSERT(!static_type.IsMalformed());
 
-  // If the static type of the value is void, the only allowed value is null,
-  // which must be verified by the type test.
-  // TODO(regis): Eliminate the test if the value is constant null.
+  // If the static type of the value is void, we are type checking the result of
+  // a void function, which was checked to be null at the return statement
+  // inside the function.
   if (static_type.IsVoidType()) {
-    return false;
+    return true;
   }
 
   // If the static type of the value is NullType, the type test is eliminated.
   // There are only three instances that can be of Class Null:
   // Object::null(), Object::sentinel(), and Object::transition_sentinel().
   // The inline code and run time code performing the type check will never
   // encounter the 2 sentinel values. The type check of a sentinel value
   // will always be eliminated here, because these sentinel values can only
   // be encountered as constants, never as actual value of a heap object
   // being type checked.
@@ skipping 2281 matching lines @@
   char* chars = reinterpret_cast<char*>(
       Isolate::Current()->current_zone()->Allocate(len));
   OS::SNPrint(chars, len, kFormat, function_name, reason);
   const Error& error = Error::Handle(
       LanguageError::New(String::Handle(String::New(chars))));
   Isolate::Current()->long_jump_base()->Jump(1, error);
 }
 
 
 }  // namespace dart