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diff --git a/Source/WTF/wtf/Function.h b/Source/WTF/wtf/Function.h
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+/*
+ * Copyright (C) 2016 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.
+ */
+
+#pragma once
+
+#include <memory>
+#include <wtf/FastMalloc.h>
+
+namespace WTF {
+
+template<typename> class Function;
+
+template <typename Out, typename... In>
+class Function<Out(In...)> {
+public:
+    Function() = default;
+    Function(std::nullptr_t) { }
+
+    template<typename CallableType, class = typename std::enable_if<std::is_rvalue_reference<CallableType&&>::value>::type>
+    Function(CallableType&& callable)
+        : m_callableWrapper(std::make_unique<CallableWrapper<CallableType>>(WTFMove(callable)))
+    {
+    }
+
+    Out operator()(In... in) const
+    {
+        if (m_callableWrapper)
+            return m_callableWrapper->call(std::forward<In>(in)...);
+        return Out();
+    }
+
+    explicit operator bool() const { return !!m_callableWrapper; }
+
+    template<typename CallableType, class = typename std::enable_if<std::is_rvalue_reference<CallableType&&>::value>::type>
+    Function& operator=(CallableType&& callable)
+    {
+        m_callableWrapper = std::make_unique<CallableWrapper<CallableType>>(WTFMove(callable));
+        return *this;
+    }
+
+    Function& operator=(std::nullptr_t)
+    {
+        m_callableWrapper = nullptr;
+        return *this;
+    }
+
+private:
+    class CallableWrapperBase {
+        WTF_MAKE_FAST_ALLOCATED;
+    public:
+        virtual ~CallableWrapperBase() { }
+
+        virtual Out call(In...) = 0;
+    };
+
+    template<typename CallableType>
+    class CallableWrapper : public CallableWrapperBase {
+    public:
+        explicit CallableWrapper(CallableType&& callable)
+            : m_callable(WTFMove(callable))
+        {
+        }
+
+        CallableWrapper(const CallableWrapper&) = delete;
+        CallableWrapper& operator=(const CallableWrapper&) = delete;
+
+        Out call(In... in) final { return m_callable(std::forward<In>(in)...); }
+
+    private:
+        CallableType m_callable;
+    };
+
+    std::unique_ptr<CallableWrapperBase> m_callableWrapper;
+};
+
+} // namespace WTF
+
+using WTF::Function;