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diff --git a/Source/WTF/wtf/Expected.h b/Source/WTF/wtf/Expected.h
new file mode 100644
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+++ b/Source/WTF/wtf/Expected.h
@@ -0,0 +1,456 @@
+/*
+ * 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.
+ */
+
+// WTF::Expected is based on std::expected, as described here: http://wg21.link/p0323r1
+// The specification expects to throw. This implementation doesn't support exceptions, uses RELEASE_ASSERT instead.
+
+#pragma once
+
+#include <cstdlib>
+#include <functional>
+#include <initializer_list>
+#include <type_traits>
+#include <utility>
+#include <wtf/Assertions.h>
+#include <wtf/Compiler.h>
+#include <wtf/Optional.h>
+#include <wtf/StdLibExtras.h>
+
+namespace WTF {
+
+template <class E>
+class UnexpectedType {
+public:
+    UnexpectedType() = delete;
+    constexpr explicit UnexpectedType(const E& e) : val(e) { }
+    constexpr explicit UnexpectedType(E&& e) : val(std::forward<E>(e)) { }
+    constexpr const E& value() const& { return val; }
+    RELAXED_CONSTEXPR E& value() & { return val; }
+    RELAXED_CONSTEXPR E&& value() && { return WTFMove(val); }
+
+private:
+    E val;
+};
+
+template <class E> constexpr bool operator==(const UnexpectedType<E>& lhs, const UnexpectedType<E>& rhs) { return lhs.value() == rhs.value(); }
+template <class E> constexpr bool operator!=(const UnexpectedType<E>& lhs, const UnexpectedType<E>& rhs) { return lhs.value() != rhs.value(); }
+template <class E> constexpr bool operator<(const UnexpectedType<E>& lhs, const UnexpectedType<E>& rhs) { return lhs.value() < rhs.value(); }
+template <class E> constexpr bool operator>(const UnexpectedType<E>& lhs, const UnexpectedType<E>& rhs) { return lhs.value() > rhs.value(); }
+template <class E> constexpr bool operator<=(const UnexpectedType<E>& lhs, const UnexpectedType<E>& rhs) { return lhs.value() <= rhs.value(); }
+template <class E> constexpr bool operator>=(const UnexpectedType<E>& lhs, const UnexpectedType<E>& rhs) { return lhs.value() >= rhs.value(); }
+
+template <class E> constexpr UnexpectedType<std::decay_t<E>> makeUnexpected(E&& v) { return UnexpectedType<typename std::decay<E>::type>(std::forward<E>(v)); }
+
+struct UnexpectTag {
+    UnexpectTag() = delete;
+};
+constexpr UnexpectTag Unexpect { };
+
+namespace ExpectedDetail {
+
+// Invoked where std::Expected would instead throw.
+inline NO_RETURN_DUE_TO_CRASH void Throw() { RELEASE_ASSERT_NOT_REACHED(); }
+
+static constexpr enum class ValueTagType { } ValueTag { };
+static constexpr enum class ErrorTagType { } ErrorTag { };
+
+template<class T, std::enable_if_t<std::is_trivially_destructible<T>::value>* = nullptr> void destroy(T&) { }
+template<class T, std::enable_if_t<!std::is_trivially_destructible<T>::value && (std::is_class<T>::value || std::is_union<T>::value)>* = nullptr> void destroy(T& t) { t.~T(); }
+
+template <class T, class E>
+union ConstexprStorage {
+    typedef T ValueType;
+    typedef E ErrorType;
+    char dummy;
+    ValueType val;
+    ErrorType err;
+    constexpr ConstexprStorage() : dummy() { }
+    constexpr ConstexprStorage(ValueTagType) : val() { }
+    constexpr ConstexprStorage(ErrorTagType) : err() { }
+    constexpr ConstexprStorage(ValueTagType, const ValueType& v) : val(v) { }
+    constexpr ConstexprStorage(ErrorTagType, const ErrorType& e) : err(e) { }
+    ~ConstexprStorage() = default;
+};
+
+template <class T, class E>
+union Storage {
+    typedef T ValueType;
+    typedef E ErrorType;
+    char dummy;
+    ValueType val;
+    ErrorType err;
+    constexpr Storage() : dummy() { }
+    constexpr Storage(ValueTagType) : val() { }
+    constexpr Storage(ErrorTagType) : err() { }
+    constexpr Storage(ValueTagType, const ValueType& val) : val(val) { }
+    constexpr Storage(ValueTagType, ValueType&& val) : val(std::forward<ValueType>(val)) { }
+    constexpr Storage(ErrorTagType, const ErrorType& err) : err(err) { }
+    constexpr Storage(ErrorTagType, ErrorType&& err) : err(std::forward<ErrorType>(err)) { }
+    ~Storage() { }
+};
+
+template <class E>
+union ConstexprStorage<void, E> {
+    typedef void ValueType;
+    typedef E ErrorType;
+    char dummy;
+    ErrorType err;
+    constexpr ConstexprStorage() : dummy() { }
+    constexpr ConstexprStorage(ValueTagType) : dummy() { }
+    constexpr ConstexprStorage(ErrorTagType) : err() { }
+    constexpr ConstexprStorage(ErrorTagType, const ErrorType& e) : err(e) { }
+    ~ConstexprStorage() = default;
+};
+
+template <class E>
+union Storage<void, E> {
+    typedef void ValueType;
+    typedef E ErrorType;
+    char dummy;
+    ErrorType err;
+    constexpr Storage() : dummy() { }
+    constexpr Storage(ValueTagType) : dummy() { }
+    constexpr Storage(ErrorTagType) : err() { }
+    constexpr Storage(ErrorTagType, const ErrorType& err) : err(err) { }
+    constexpr Storage(ErrorTagType, ErrorType&& err) : err(std::forward<ErrorType>(err)) { }
+    ~Storage() { }
+};
+
+template <class T, class E>
+struct ConstexprBase {
+    typedef T ValueType;
+    typedef E ErrorType;
+    ConstexprStorage<ValueType, ErrorType> s;
+    bool has;
+    constexpr ConstexprBase() : s(), has(true) { }
+    constexpr ConstexprBase(ValueTagType tag) : s(tag), has(true) { }
+    constexpr ConstexprBase(ErrorTagType tag) : s(tag), has(false) { }
+    constexpr ConstexprBase(ValueTagType tag, const ValueType& val) : s(tag, val), has(true) { }
+    constexpr ConstexprBase(ErrorTagType tag, const ErrorType& err) : s(tag, err), has(false) { }
+    ~ConstexprBase() = default;
+};
+
+template <class T, class E>
+struct Base {
+    typedef T ValueType;
+    typedef E ErrorType;
+    Storage<ValueType, ErrorType> s;
+    bool has;
+    constexpr Base() : s(), has(true) { }
+    constexpr Base(ValueTagType tag) : s(tag), has(true) { }
+    constexpr Base(ErrorTagType tag) : s(tag), has(false) { }
+    constexpr Base(ValueTagType tag, const ValueType& val) : s(tag, val), has(true) { }
+    constexpr Base(ValueTagType tag, ValueType&& val) : s(tag, std::forward<ValueType>(val)), has(true) { }
+    constexpr Base(ErrorTagType tag, const ErrorType& err) : s(tag, err), has(false) { }
+    constexpr Base(ErrorTagType tag, ErrorType&& err) : s(tag, std::forward<ErrorType>(err)), has(false) { }
+    Base(const Base& o)
+        : has(o.has)
+    {
+        if (has)
+            ::new (&s.val) ValueType(o.s.val);
+        else
+            ::new (&s.err) ErrorType(o.s.err);
+    }
+    Base(Base&& o)
+        : has(o.has)
+    {
+        if (has)
+            ::new (&s.val) ValueType(WTFMove(o.s.val));
+        else
+            ::new (&s.err) ErrorType(WTFMove(o.s.err));
+    }
+    ~Base()
+    {
+        if (has)
+            destroy(s.val);
+        else
+            destroy(s.err);
+    }
+};
+
+template <class E>
+struct ConstexprBase<void, E> {
+    typedef void ValueType;
+    typedef E ErrorType;
+    ConstexprStorage<ValueType, ErrorType> s;
+    bool has;
+    constexpr ConstexprBase() : s(), has(true) { }
+    constexpr ConstexprBase(ValueTagType tag) : s(tag), has(true) { }
+    constexpr ConstexprBase(ErrorTagType tag) : s(tag), has(false) { }
+    constexpr ConstexprBase(ErrorTagType tag, const ErrorType& err) : s(tag, err), has(false) { }
+    constexpr ConstexprBase(ErrorTagType tag, ErrorType&& err) : s(tag, std::forward<ErrorType>(err)), has(false) { }
+    ~ConstexprBase() = default;
+};
+
+template <class E>
+struct Base<void, E> {
+    typedef void ValueType;
+    typedef E ErrorType;
+    Storage<ValueType, ErrorType> s;
+    bool has;
+    constexpr Base() : s(), has(true) { }
+    constexpr Base(ValueTagType tag) : s(tag), has(true) { }
+    constexpr Base(ErrorTagType tag) : s(tag), has(false) { }
+    constexpr Base(ErrorTagType tag, const ErrorType& err) : s(tag, err), has(false) { }
+    constexpr Base(ErrorTagType tag, ErrorType&& err) : s(tag, std::forward<ErrorType>(err)), has(false) { }
+    Base(const Base& o)
+        : has(o.has)
+    {
+        if (!has)
+            ::new (&s.err) ErrorType(o.s.err);
+    }
+    Base(Base&& o)
+        : has(o.has)
+    {
+        if (!has)
+            ::new (&s.err) ErrorType(WTFMove(o.s.err));
+    }
+    ~Base()
+    {
+        if (!has)
+            destroy(s.err);
+    }
+};
+
+template <class T, class E>
+using BaseSelect = typename std::conditional<
+    ((std::is_void<T>::value || std::is_trivially_destructible<T>::value)
+        && std::is_trivially_destructible<E>::value),
+    ConstexprBase<typename std::remove_const<T>::type, typename std::remove_const<E>::type>,
+    Base<typename std::remove_const<T>::type, typename std::remove_const<E>::type>
+>::type;
+
+} // namespace ExpectedDetail
+
+template <class T, class E>
+class Expected : private ExpectedDetail::BaseSelect<T, E> {
+    typedef ExpectedDetail::BaseSelect<T, E> base;
+
+public:
+    typedef typename base::ValueType ValueType;
+    typedef typename base::ErrorType ErrorType;
+
+private:
+    typedef Expected<ValueType, ErrorType> type;
+
+public:
+    // template <class U> struct rebind { using type = Expected<U, ErrorType>; };
+
+    constexpr Expected() : base(ExpectedDetail::ValueTag) { }
+    Expected(const Expected&) = default;
+    Expected(Expected&&) = default;
+    constexpr Expected(const ValueType& e) : base(ExpectedDetail::ValueTag, e) { }
+    constexpr Expected(ValueType&& e) : base(ExpectedDetail::ValueTag, std::forward<ValueType>(e)) { }
+    // template <class... Args> constexpr explicit Expected(in_place_t, Args&&...);
+    // template <class U, class... Args> constexpr explicit Expected(in_place_t, std::initializer_list<U>, Args&&...);
+    constexpr Expected(UnexpectedType<ErrorType> const& u) : base(ExpectedDetail::ErrorTag, u.value()) { }
+    constexpr Expected(UnexpectedType<ErrorType>&& u) : base(ExpectedDetail::ErrorTag, std::forward<UnexpectedType<E>>(u).value()) { }
+    template <class Err> constexpr Expected(UnexpectedType<Err> const& u) : base(ExpectedDetail::ErrorTag, u.value()) { }
+    // template <class... Args> constexpr explicit Expected(UnexpectTag, Args&&...);
+    // template <class U, class... Args> constexpr explicit Expected(UnexpectTag, std::initializer_list<U>, Args&&...);
+
+    ~Expected() = default;
+
+    Expected& operator=(const Expected& e) { type(e).swap(*this); return *this; }
+    Expected& operator=(Expected&& e) { type(WTFMove(e)).swap(*this); return *this; }
+    template <class U> Expected& operator=(U&& u) { type(WTFMove(u)).swap(*this); return *this; }
+    Expected& operator=(const UnexpectedType<ErrorType>& u) { type(u).swap(*this); return *this; }
+    Expected& operator=(UnexpectedType<ErrorType>&& u) { type(WTFMove(u)).swap(*this); return *this; }
+    // template <class... Args> void emplace(Args&&...);
+    // template <class U, class... Args> void emplace(std::initializer_list<U>, Args&&...);
+
+    void swap(Expected& o)
+    {
+        using std::swap;
+        if (base::has && o.has)
+            swap(base::s.val, o.s.val);
+        else if (base::has && !o.has) {
+            ErrorType e(WTFMove(o.s.err));
+            ExpectedDetail::destroy(o.s.err);
+            ::new (&o.s.val) ValueType(WTFMove(base::s.val));
+            ExpectedDetail::destroy(base::s.val);
+            ::new (&base::s.err) ErrorType(WTFMove(e));
+            swap(base::has, o.has);
+        } else if (!base::has && o.has) {
+            ValueType v(WTFMove(o.s.val));
+            ExpectedDetail::destroy(o.s.val);
+            ::new (&o.s.err) ErrorType(WTFMove(base::s.err));
+            ExpectedDetail::destroy(base::s.err);
+            ::new (&base::s.val) ValueType(WTFMove(v));
+            swap(base::has, o.has);
+        } else
+            swap(base::s.err, o.s.err);
+    }
+
+    constexpr const ValueType* operator->() const { return &base::s.val; }
+    ValueType* operator->() { return &base::s.val; }
+    constexpr const ValueType& operator*() const & { return base::s.val; }
+    ValueType& operator*() & { return base::s.val; }
+    constexpr const ValueType&& operator*() const && { return WTFMove(base::s.val); }
+    RELAXED_CONSTEXPR ValueType&& operator*() && { return WTFMove(base::s.val); }
+    constexpr explicit operator bool() const { return base::has; }
+    constexpr bool hasValue() const { return base::has; }
+    constexpr const ValueType& value() const & { return base::has ? base::s.val : (ExpectedDetail::Throw(), base::s.val); }
+    RELAXED_CONSTEXPR ValueType& value() & { return base::has ? base::s.val : (ExpectedDetail::Throw(), base::s.val); }
+    constexpr const ValueType&& value() const && { return base::has ? base::s.val : (ExpectedDetail::Throw(), base::s.val); }
+    RELAXED_CONSTEXPR ValueType&& value() && { return WTFMove(base::has ? base::s.val : (ExpectedDetail::Throw(), base::s.val)); }
+    constexpr const ErrorType& error() const & { return !base::has ? base::s.err : (ExpectedDetail::Throw(), base::s.err); }
+    ErrorType& error() & { return !base::has ? base::s.err : (ExpectedDetail::Throw(), base::s.err); }
+    RELAXED_CONSTEXPR ErrorType&& error() && { return !base::has ? base::s.err : (ExpectedDetail::Throw(), base::s.err); }
+    constexpr const ErrorType&& error() const && { return !base::has ? base::s.err : (ExpectedDetail::Throw(), base::s.err); }
+    constexpr UnexpectedType<ErrorType> getUnexpected() const { return UnexpectedType<ErrorType>(base::s.err); }
+    template <class U> constexpr ValueType valueOr(U&& u) const & { return base::has ? **this : static_cast<ValueType>(std::forward<U>(u)); }
+    template <class U> ValueType valueOr(U&& u) && { return base::has ? WTFMove(**this) : static_cast<ValueType>(std::forward<U>(u)); }
+};
+
+template <class E>
+class Expected<void, E> : private ExpectedDetail::BaseSelect<void, E> {
+    typedef ExpectedDetail::BaseSelect<void, E> base;
+
+public:
+    typedef typename base::ValueType ValueType;
+    typedef typename base::ErrorType ErrorType;
+
+private:
+    typedef Expected<ValueType, ErrorType> type;
+
+public:
+    // template <class U> struct rebind { typedef Expected<U, ErrorType> type; };
+
+    constexpr Expected() : base(ExpectedDetail::ValueTag) { }
+    Expected(const Expected&) = default;
+    Expected(Expected&&) = default;
+    // constexpr explicit Expected(in_place_t);
+    constexpr Expected(UnexpectedType<E> const& u) : base(ExpectedDetail::ErrorTag, u.value()) { }
+    constexpr Expected(UnexpectedType<E>&& u) : base(ExpectedDetail::ErrorTag, std::forward<UnexpectedType<E>>(u).value()) { }
+    template <class Err> constexpr Expected(UnexpectedType<Err> const& u) : base(ExpectedDetail::ErrorTag, u.value()) { }
+
+    ~Expected() = default;
+
+    Expected& operator=(const Expected& e) { type(e).swap(*this); return *this; }
+    Expected& operator=(Expected&& e) { type(WTFMove(e)).swap(*this); return *this; }
+    Expected& operator=(const UnexpectedType<E>& u) { type(u).swap(*this); return *this; } // Not in the current paper.
+    Expected& operator=(UnexpectedType<E>&& u) { type(WTFMove(u)).swap(*this); return *this; } // Not in the current paper.
+    // void emplace();
+
+    void swap(Expected& o)
+    {
+        using std::swap;
+        if (base::has && o.has) {
+            // Do nothing.
+        } else if (base::has && !o.has) {
+            ErrorType e(WTFMove(o.s.err));
+            ::new (&base::s.err) ErrorType(e);
+            swap(base::has, o.has);
+        } else if (!base::has && o.has) {
+            ::new (&o.s.err) ErrorType(WTFMove(base::s.err));
+            swap(base::has, o.has);
+        } else
+            swap(base::s.err, o.s.err);
+    }
+
+    constexpr explicit operator bool() const { return base::has; }
+    constexpr bool hasValue() const { return base::has; }
+    void value() const { !base::has ? ExpectedDetail::Throw() : void(); }
+    constexpr const E& error() const & { return !base::has ? base::s.err : (ExpectedDetail::Throw(), base::s.err); }
+    E& error() & { return !base::has ? base::s.err : (ExpectedDetail::Throw(), base::s.err); } // Not in the current paper.
+    RELAXED_CONSTEXPR E&& error() && { return !base::has ? base::s.err : (ExpectedDetail::Throw(), base::s.err); }
+    constexpr const E&& error() const && { return !base::has ? base::s.err : (ExpectedDetail::Throw(), base::s.err); } // Not in the current paper.
+    // constexpr E& error() &;
+    constexpr UnexpectedType<E> getUnexpected() const { return UnexpectedType<E>(base::s.err); }
+};
+
+template <class T, class E> constexpr bool operator==(const Expected<T, E>& x, const Expected<T, E>& y) { return bool(x) == bool(y) && (x ? x.value() == y.value() : x.error() == y.error()); }
+template <class T, class E> constexpr bool operator!=(const Expected<T, E>& x, const Expected<T, E>& y) { return !(x == y); }
+template <class T, class E> constexpr bool operator<(const Expected<T, E>& x, const Expected<T, E>& y) { return (!bool(x) && bool(y)) ? false : ((bool(x) && !bool(y)) ? true : ((bool(x) && bool(y)) ? x.value() < y.value() : x.error() < y.error())); }
+template <class T, class E> constexpr bool operator>(const Expected<T, E>& x, const Expected<T, E>& y) { return !(x == y) && !(x < y); }
+template <class T, class E> constexpr bool operator<=(const Expected<T, E>& x, const Expected<T, E>& y) { return (x == y) || (x < y); }
+template <class T, class E> constexpr bool operator>=(const Expected<T, E>& x, const Expected<T, E>& y) { return (x == y) || (x > y); }
+
+template <class E> constexpr bool operator==(const Expected<void, E>& x, const Expected<void, E>& y) { return bool(x) == bool(y) && (x ? true : x.error() == y.error()); } // Not in the current paper.
+template <class E> constexpr bool operator<(const Expected<void, E>& x, const Expected<void, E>& y) { return (!bool(x) && bool(y)) ? false : ((bool(x) && !bool(y)) ? true : ((bool(x) && bool(y)) ? false : x.error() < y.error())); } // Not in the current paper.
+
+template <class T, class E> constexpr bool operator==(const Expected<T, E>& x, const T& y) { return x == Expected<T, E>(y); }
+template <class T, class E> constexpr bool operator==(const T& x, const Expected<T, E>& y) { return Expected<T, E>(x) == y; }
+template <class T, class E> constexpr bool operator!=(const Expected<T, E>& x, const T& y) { return x != Expected<T, E>(y); }
+template <class T, class E> constexpr bool operator!=(const T& x, const Expected<T, E>& y) { return Expected<T, E>(x) != y; }
+template <class T, class E> constexpr bool operator<(const Expected<T, E>& x, const T& y) { return x < Expected<T, E>(y); }
+template <class T, class E> constexpr bool operator<(const T& x, const Expected<T, E>& y) { return Expected<T, E>(x) < y; }
+template <class T, class E> constexpr bool operator<=(const Expected<T, E>& x, const T& y) { return x <= Expected<T, E>(y); }
+template <class T, class E> constexpr bool operator<=(const T& x, const Expected<T, E>& y) { return Expected<T, E>(x) <= y; }
+template <class T, class E> constexpr bool operator>(const Expected<T, E>& x, const T& y) { return x > Expected<T, E>(y); }
+template <class T, class E> constexpr bool operator>(const T& x, const Expected<T, E>& y) { return Expected<T, E>(x) > y; }
+template <class T, class E> constexpr bool operator>=(const Expected<T, E>& x, const T& y) { return x >= Expected<T, E>(y); }
+template <class T, class E> constexpr bool operator>=(const T& x, const Expected<T, E>& y) { return Expected<T, E>(x) >= y; }
+
+template <class T, class E> constexpr bool operator==(const Expected<T, E>& x, const UnexpectedType<E>& y) { return x == Expected<T, E>(y); }
+template <class T, class E> constexpr bool operator==(const UnexpectedType<E>& x, const Expected<T, E>& y) { return Expected<T, E>(x) == y; }
+template <class T, class E> constexpr bool operator!=(const Expected<T, E>& x, const UnexpectedType<E>& y) { return x != Expected<T, E>(y); }
+template <class T, class E> constexpr bool operator!=(const UnexpectedType<E>& x, const Expected<T, E>& y) { return Expected<T, E>(x) != y; }
+template <class T, class E> constexpr bool operator<(const Expected<T, E>& x, const UnexpectedType<E>& y) { return x < Expected<T, E>(y); }
+template <class T, class E> constexpr bool operator<(const UnexpectedType<E>& x, const Expected<T, E>& y) {  return Expected<T, E>(x) < y; }
+template <class T, class E> constexpr bool operator<=(const Expected<T, E>& x, const UnexpectedType<E>& y) { return x <= Expected<T, E>(y); }
+template <class T, class E> constexpr bool operator<=(const UnexpectedType<E>& x, const Expected<T, E>& y) { return Expected<T, E>(x) <= y; }
+template <class T, class E> constexpr bool operator>(const Expected<T, E>& x, const UnexpectedType<E>& y) { return x > Expected<T, E>(y); }
+template <class T, class E> constexpr bool operator>(const UnexpectedType<E>& x, const Expected<T, E>& y) { return Expected<T, E>(x) > y; }
+template <class T, class E> constexpr bool operator>=(const Expected<T, E>& x, const UnexpectedType<E>& y) { return x >= Expected<T, E>(y); }
+template <class T, class E> constexpr bool operator>=(const UnexpectedType<E>& x, const Expected<T, E>& y) { return Expected<T, E>(x) >= y; }
+
+template <typename T, typename E> void swap(Expected<T, E>& x, Expected<T, E>& y) { x.swap(y); }
+
+template <class T, class E = std::nullopt_t> constexpr Expected<std::decay_t<T>, E> makeExpected(T&& v)
+{
+    return Expected<typename std::decay<T>::type, E>(std::forward<T>(v));
+}
+template <class T, class E> constexpr Expected<T, std::decay_t<E>> makeExpectedFromError(E&& e) { return Expected<T, std::decay_t<E>>(makeUnexpected(e)); }
+template <class T, class E, class U> constexpr Expected<T, E> makeExpectedFromError(U&& u) { return Expected<T, E>(makeUnexpected(E { std::forward<U>(u) } )); }
+// template <class F, class E = std::nullopt_t> constexpr Expected<typename std::result_of<F>::type, E> makeExpected_from_call(F f);
+
+inline Expected<void, std::nullopt_t> makeExpected() { return Expected<void, std::nullopt_t>(); }
+
+} // namespace WTF
+
+namespace std {
+
+template <class T, class E> struct hash<WTF::Expected<T, E>> {
+    typedef WTF::Expected<T, E> argument_type;
+    typedef std::size_t result_type;
+    result_type operator()(argument_type const& e) const { return e ? hash<typename argument_type::ValueType> { } (e.value()) : hash<typename argument_type::ErrorType> { } (e.error()); }
+};
+
+template <class E> struct hash<WTF::Expected<void, E>> {
+    typedef WTF::Expected<void, E> argument_type;
+    typedef std::size_t result_type;
+    result_type operator()(argument_type const& e) const { return e ? 0 : hash<typename argument_type::ErrorType> { } (e.error()); }
+};
+
+}
+
+using WTF::UnexpectedType;
+using WTF::makeUnexpected;
+using WTF::Unexpect;
+using WTF::Expected;
+using WTF::makeExpected;
+using WTF::makeExpectedFromError;