permit `co_return {};` in a coro::Task<T>-returning coroutine to default-construct a T

Summary: In a coroutine returning a `coro::Task<T>`, make `co_return {};` return a default-constructed `T` rather than a default-constructed `folly::Try`.

Reviewed By: yfeldblum, andriigrynenko

Differential Revision: D18466785

fbshipit-source-id: ed20f0f5ad98a0ef8e693e490417a7e36f706822

folly/experimental/coro/Task.h

@@ -134,27 +134,21 @@ class TaskPromise : public TaskPromiseBase {
         exception_wrapper::from_exception_ptr(std::current_exception()));
   }
 
-  template <typename U>
-  void return_value(U&& value) {
-    static_assert(
-        std::is_convertible<U&&, StorageType>::value,
-        "cannot convert return value to type T");
-    result_.emplace(static_cast<U&&>(value));
+  void return_value(T&& t) {
+    result_.emplace(static_cast<T&&>(t));
   }
 
-  void return_value(const Try<StorageType>& t) {
-    DCHECK(t.hasValue() || t.hasException());
-    result_ = t;
-  }
-
-  void return_value(Try<StorageType>& t) {
-    DCHECK(t.hasValue() || t.hasException());
-    result_ = t;
-  }
-
-  void return_value(Try<StorageType>&& t) {
-    DCHECK(t.hasValue() || t.hasException());
-    result_ = std::move(t);
+  template <typename U>
+  void return_value(U&& value) {
+    if constexpr (std::is_same_v<remove_cvref_t<U>, Try<StorageType>>) {
+      DCHECK(value.hasValue() || value.hasException());
+      result_ = static_cast<U&&>(value);
+    } else {
+      static_assert(
+          std::is_convertible<U&&, StorageType>::value,
+          "cannot convert return value to type T");
+      result_.emplace(static_cast<U&&>(value));
+    }
   }
 
   Try<StorageType>& result() {

folly/experimental/coro/test/CoroTest.cpp

@@ -37,6 +37,14 @@
 
 using namespace folly;
 
+struct S {
+  int x = 42;
+};
+
+coro::Task<S> taskS() {
+  co_return {};
+}
+
 coro::Task<int> task42() {
   co_return 42;
 }
@@ -557,4 +565,11 @@ TEST(Coro, CancellableSleep) {
   CHECK((end - start) < 1s);
 }
 
+TEST(Coro, DefaultConstructible) {
+  coro::blockingWait([]() -> coro::Task<void> {
+    auto s = co_await taskS();
+    EXPECT_EQ(42, s.x);
+  }());
+}
+
 #endif