+template <class U, class V>
+bool operator==(const Optional<U>& a, const V& b) {
+ return a.hasValue() && a.value() == b;
+}
+
+template <class U, class V>
+bool operator!=(const Optional<U>& a, const V& b) {
+ return !(a == b);
+}
+
+template <class U, class V>
+bool operator==(const U& a, const Optional<V>& b) {
+ return b.hasValue() && b.value() == a;
+}
+
+template <class U, class V>
+bool operator!=(const U& a, const Optional<V>& b) {
+ return !(a == b);
+}
+
+template <class U, class V>
+bool operator==(const Optional<U>& a, const Optional<V>& b) {
+ if (a.hasValue() != b.hasValue()) { return false; }
+ if (a.hasValue()) { return a.value() == b.value(); }
+ return true;
+}
+
+template <class U, class V>
+bool operator!=(const Optional<U>& a, const Optional<V>& b) {
+ return !(a == b);
+}
+
+template <class U, class V>
+bool operator<(const Optional<U>& a, const Optional<V>& b) {
+ if (a.hasValue() != b.hasValue()) { return a.hasValue() < b.hasValue(); }
+ if (a.hasValue()) { return a.value() < b.value(); }
+ return false;
+}
+
+template <class U, class V>
+bool operator>(const Optional<U>& a, const Optional<V>& b) {
+ return b < a;
+}
+
+template <class U, class V>
+bool operator<=(const Optional<U>& a, const Optional<V>& b) {
+ return !(b < a);
+}
+
+template <class U, class V>
+bool operator>=(const Optional<U>& a, const Optional<V>& b) {
+ return !(a < b);
+}
+
+// Suppress comparability of Optional<T> with T, despite implicit conversion.
+template<class V> bool operator< (const Optional<V>&, const V& other) = delete;
+template<class V> bool operator<=(const Optional<V>&, const V& other) = delete;
+template<class V> bool operator>=(const Optional<V>&, const V& other) = delete;
+template<class V> bool operator> (const Optional<V>&, const V& other) = delete;
+template<class V> bool operator< (const V& other, const Optional<V>&) = delete;
+template<class V> bool operator<=(const V& other, const Optional<V>&) = delete;
+template<class V> bool operator>=(const V& other, const Optional<V>&) = delete;
+template<class V> bool operator> (const V& other, const Optional<V>&) = delete;
+
+///////////////////////////////////////////////////////////////////////////////
+
+} // namespace folly
+
+// Allow usage of Optional<T> in std::unordered_map and std::unordered_set
+FOLLY_NAMESPACE_STD_BEGIN
+template <class T>
+struct hash<folly::Optional<T>> {
+ size_t operator()(folly::Optional<T> const& obj) const {
+ if (!obj.hasValue()) {
+ return 0;
+ }
+ return hash<typename remove_const<T>::type>()(*obj);
+ }
+};
+FOLLY_NAMESPACE_STD_END