Reland: Templatize visitor arguments for TraceTrait mark and trace methods.

Source/wtf/TypeTraits.h

  /*
  * Copyright (C) 2006, 2007, 2008 Apple Inc. All rights reserved.
  * Copyright (C) 2009, 2010 Google Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 285 matching lines @@
 #define EnsurePtrConvertibleArgDecl(From, To) \
     typename WTF::EnableIf<WTF::IsPointerConvertible<From, To>::Value, bool>::Type = true
 #define EnsurePtrConvertibleArgDefn(From, To) \
     typename WTF::EnableIf<WTF::IsPointerConvertible<From, To>::Value, bool>::Type
 
 } // namespace WTF
 
 namespace blink {
 
 class JSONValue;
+class Visitor;
 
 } // namespace blink
 
 namespace WTF {
 
     // FIXME: Disable pointer conversion checking against JSONValue.
     // The current CodeGeneratorInspector.py generates code which upcasts to JSONValue from undefined types.
     template<typename From> class IsPointerConvertible<From, blink::JSONValue> {
     public:
         enum {
             Value = true
         };
     };
 
+#if COMPILER(MSVC)
+template<typename T, bool = __is_class(T)> struct NeedsTracingMSVC;
+
+template<typename T>
+struct NeedsTracingMSVC<T, true> {
+    __if_exists(T::trace)
+    {
+        static const bool value = true;
+    }
+    __if_not_exists(T::trace)
+    {
+        static const bool value = false;
+    }
+};
+
+template<typename T>
+struct NeedsTracingMSVC<T, false> {
+    static const bool value = false;
+};
+#endif
+
 template<typename T>
 class NeedsTracing {
+#if COMPILER(MSVC)
+public:
+    static const bool value = NeedsTracingMSVC<T>::value;
+#else
     typedef char YesType;
     typedef struct NoType {
         char padding[8];
     } NoType;
-#if COMPILER(MSVC)
-    template<typename V> static YesType checkHasTraceMethod(char[&V::trace != 0]);
-#else
     template<size_t> struct HasMethod;
-    template<typename V> static YesType checkHasTraceMethod(HasMethod<sizeof(&V::trace)>*);
-#endif // COMPILER(MSVC)
+    template<typename V> static YesType checkHasTraceMethod(HasMethod<sizeof(static_cast<void (V::*)(blink::Visitor*)>(&V::trace))>*);
     template<typename V> static NoType checkHasTraceMethod(...);
 public:
     // We add sizeof(T) to both sides here, because we want it to fail for
     // incomplete types. Otherwise it just assumes that incomplete types do not
     // have a trace method, which may not be true.
-    static const bool value = sizeof(YesType) + sizeof(T) == sizeof(checkHasTraceMethod<T>(0)) + sizeof(T);
+    static const bool value = sizeof(YesType) + sizeof(T) == sizeof(checkHasTraceMethod<T>(nullptr)) + sizeof(T);
+#endif // COMPILER(MSVC)
 };
 
 // Convenience template wrapping the NeedsTracingLazily template in
 // Collection Traits. It helps make the code more readable.
 template<typename Traits>
 class ShouldBeTraced {
 public:
     static const bool value = Traits::template NeedsTracingLazily<>::value;
 };
 
 template<typename T, typename U>
 struct NeedsTracing<std::pair<T, U> > {
     static const bool value = NeedsTracing<T>::value || NeedsTracing<U>::value || IsWeak<T>::value || IsWeak<U>::value;
 };
 
 } // namespace WTF
 
 #endif // TypeTraits_h