Fix IsBaseOf in output_traits.h.

ppapi/cpp/output_traits.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef PPAPI_CPP_OUTPUT_TRAITS_H_
 #define PPAPI_CPP_OUTPUT_TRAITS_H_
 
 #include "ppapi/c/pp_resource.h"
 #include "ppapi/cpp/array_output.h"
 
@@ skipping 17 matching lines @@
 // This goop is a trick used to implement a template that can be used to
 // determine if a given class is the base class of another given class. It is
 // used in the resource object partial specialization below.
 template<typename, typename> struct IsSame {
   static bool const value = false;
 };
 template<typename A> struct IsSame<A, A> {
   static bool const value = true;
 };
 template<typename Base, typename Derived> struct IsBaseOf {
+ private:
+  // This class doesn't work correctly with forward declarations.
+  // Because sizeof cannot be applied to incomplete types, this line prevents us
+  // from passing in forward declarations.
+  typedef char (*EnsureTypesAreComplete)[sizeof(Base) + sizeof(Derived)];
+
   static Derived* CreateDerived();
   static char (&Check(Base*))[1];
   static char (&Check(...))[2];
+
+ public:
   static bool const value = sizeof Check(CreateDerived()) == 1 &&
                             !IsSame<Base const, void const>::value;
 };
 
 // Template to optionally derive from a given base class T if the given
 // predicate P is true.
 template <class T, bool P> struct InheritIf {};
 template <class T> struct InheritIf<T, true> : public T {};
 
 // Single output parameters ----------------------------------------------------
@@ skipping 171 matching lines @@
   // Retrieves the underlying vector that can be passed to the plugin.
   static inline std::vector<pp::Var>& StorageToPluginArg(StorageType& t) {
     return t.output();
   }
 };
 
 }  // namespace internal
 }  // namespace pp
 
 #endif  // PPAPI_CPP_OUTPUT_TRAITS_H_