Support returning move-only types in folly::Expected::then

Summary: the codepath for .then is copying the return value in one place, which prevents using move-only types (and might incur extra costs).

Reviewed By: yfeldblum

Differential Revision: D29159637

fbshipit-source-id: 892b73266cfe45c9e09b9b648d7b7703871c4323

folly/Expected.h

@@ -499,16 +499,18 @@ inline T&& operator,(T&& t, Unit) noexcept {
 
 struct ExpectedHelper {
   template <class Error, class T>
-  static constexpr Expected<T, Error> return_(T t) {
-    return folly::makeExpected<Error>(t);
+  static constexpr Expected<typename std::decay<T>::type, Error> return_(
+      T&& t) {
+    return folly::makeExpected<Error>(static_cast<T&&>(t));
   }
+
   template <
       class Error,
       class T,
       class U FOLLY_REQUIRES_TRAILING(
           expected_detail::IsConvertible<U&&, Error>::value)>
-  static constexpr Expected<T, Error> return_(Expected<T, U> t) {
-    return t;
+  static constexpr Expected<T, Error> return_(Expected<T, U>&& t) {
+    return Expected<T, Error>(static_cast<Expected<T, U>&&>(t));
   }
 
   template <class This>

folly/test/ExpectedTest.cpp

@@ -877,4 +877,20 @@ TEST(Expected, ConstructorConstructibleNotConvertible) {
     ce = e;
   }
 }
+
+TEST(Expected, TestUnique) {
+  auto mk = []() -> Expected<std::unique_ptr<int>, int> {
+    return std::make_unique<int>(1);
+  };
+
+  EXPECT_EQ(
+      2, **mk().then([](auto r) { return std::make_unique<int>(*r + 1); }));
+
+  // Test converting errors works
+  EXPECT_EQ(
+      2, **mk().then([](auto r) -> Expected<std::unique_ptr<int>, double> {
+        return std::make_unique<int>(*r + 1);
+      }));
+}
+
 } // namespace folly