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diff --git a/Source/WTF/wtf/TypeCasts.h b/Source/WTF/wtf/TypeCasts.h
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+/*
+ * Copyright (C) 2014 Apple Inc. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY APPLE INC. AND ITS CONTRIBUTORS ``AS IS''
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
+ * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS
+ * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
+ * THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#ifndef TypeCasts_h
+#define TypeCasts_h
+
+#include <type_traits>
+
+namespace WTF {
+
+template <typename ExpectedType, typename ArgType, bool isBaseType = std::is_base_of<ExpectedType, ArgType>::value>
+struct TypeCastTraits {
+    static bool isOfType(ArgType&)
+    {
+        // If you're hitting this assertion, it is likely because you used
+        // is<>() or downcast<>() with a type that doesn't have the needed
+        // TypeCastTraits specialization. Please use the following macro
+        // to add that specialization:
+        // SPECIALIZE_TYPE_TRAITS_BEGIN() / SPECIALIZE_TYPE_TRAITS_END()
+        static_assert(std::is_void<ExpectedType>::value, "Missing TypeCastTraits specialization");
+        return false;
+    }
+};
+
+// Template specialization for the case where ExpectedType is a base of ArgType,
+// so we can return return true unconditionally.
+template <typename ExpectedType, typename ArgType>
+struct TypeCastTraits<ExpectedType, ArgType, true /* isBaseType */> {
+    static bool isOfType(ArgType&) { return true; }
+};
+
+// Type checking function, to use before casting with downcast<>().
+template <typename ExpectedType, typename ArgType>
+inline bool is(ArgType& source)
+{
+    static_assert(std::is_base_of<ArgType, ExpectedType>::value, "Unnecessary type check");
+    return TypeCastTraits<const ExpectedType, const ArgType>::isOfType(source);
+}
+
+template <typename ExpectedType, typename ArgType>
+inline bool is(ArgType* source)
+{
+    static_assert(std::is_base_of<ArgType, ExpectedType>::value, "Unnecessary type check");
+    return source && TypeCastTraits<const ExpectedType, const ArgType>::isOfType(*source);
+}
+
+// Update T's constness to match Reference's.
+template <typename Reference, typename T>
+struct match_constness {
+    typedef typename std::conditional<std::is_const<Reference>::value, typename std::add_const<T>::type, typename std::remove_const<T>::type>::type type;
+};
+
+// Safe downcasting functions.
+template<typename Target, typename Source>
+inline typename match_constness<Source, Target>::type& downcast(Source& source)
+{
+    static_assert(!std::is_same<Source, Target>::value, "Unnecessary cast to same type");
+    static_assert(std::is_base_of<Source, Target>::value, "Should be a downcast");
+    ASSERT_WITH_SECURITY_IMPLICATION(is<Target>(source));
+    return static_cast<typename match_constness<Source, Target>::type&>(source);
+}
+template<typename Target, typename Source>
+inline typename match_constness<Source, Target>::type* downcast(Source* source)
+{
+    static_assert(!std::is_same<Source, Target>::value, "Unnecessary cast to same type");
+    static_assert(std::is_base_of<Source, Target>::value, "Should be a downcast");
+    ASSERT_WITH_SECURITY_IMPLICATION(!source || is<Target>(*source));
+    return static_cast<typename match_constness<Source, Target>::type*>(source);
+}
+
+// Add support for type checking / casting using is<>() / downcast<>() helpers for a specific class.
+#define SPECIALIZE_TYPE_TRAITS_BEGIN(ClassName) \
+namespace WTF { \
+template <typename ArgType> \
+class TypeCastTraits<const ClassName, ArgType, false /* isBaseType */> { \
+public: \
+    static bool isOfType(ArgType& source) { return isType(source); } \
+private:
+
+#define SPECIALIZE_TYPE_TRAITS_END() \
+}; \
+}
+
+} // namespace WTF
+
+using WTF::TypeCastTraits;
+using WTF::is;
+using WTF::downcast;
+
+#endif // TypeCasts_h