+///////////////////////////////////////////////////////////////////////////////
+// Comparisons.
+
+template <class U, class V>
+constexpr bool operator==(const Optional<U>& a, const V& b) {
+ return a.hasValue() && a.value() == b;
+}
+
+template <class U, class V>
+constexpr bool operator!=(const Optional<U>& a, const V& b) {
+ return !(a == b);
+}
+
+template <class U, class V>
+constexpr bool operator==(const U& a, const Optional<V>& b) {
+ return b.hasValue() && b.value() == a;
+}
+
+template <class U, class V>
+constexpr bool operator!=(const U& a, const Optional<V>& b) {
+ return !(a == b);
+}
+
+template <class U, class V>
+FOLLY_CPP14_CONSTEXPR 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>
+constexpr bool operator!=(const Optional<U>& a, const Optional<V>& b) {
+ return !(a == b);
+}
+
+template <class U, class V>
+FOLLY_CPP14_CONSTEXPR 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>
+constexpr bool operator>(const Optional<U>& a, const Optional<V>& b) {
+ return b < a;
+}
+
+template <class U, class V>
+constexpr bool operator<=(const Optional<U>& a, const Optional<V>& b) {
+ return !(b < a);
+}
+
+template <class U, class V>
+constexpr 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;
+
+// Comparisons with none
+template <class V>
+constexpr bool operator==(const Optional<V>& a, None) noexcept {
+ return !a.hasValue();
+}
+template <class V>
+constexpr bool operator==(None, const Optional<V>& a) noexcept {
+ return !a.hasValue();
+}
+template <class V>
+constexpr bool operator<(const Optional<V>&, None) noexcept {
+ return false;
+}
+template <class V>
+constexpr bool operator<(None, const Optional<V>& a) noexcept {
+ return a.hasValue();
+}
+template <class V>
+constexpr bool operator>(const Optional<V>& a, None) noexcept {
+ return a.hasValue();
+}
+template <class V>
+constexpr bool operator>(None, const Optional<V>&) noexcept {
+ return false;
+}
+template <class V>
+constexpr bool operator<=(None, const Optional<V>&) noexcept {
+ return true;
+}
+template <class V>
+constexpr bool operator<=(const Optional<V>& a, None) noexcept {
+ return !a.hasValue();
+}
+template <class V>
+constexpr bool operator>=(const Optional<V>&, None) noexcept {
+ return true;
+}
+template <class V>
+constexpr bool operator>=(None, const Optional<V>& a) noexcept {
+ return !a.hasValue();
+}
+
+///////////////////////////////////////////////////////////////////////////////
+