Fix SharedPromise<void>::setValue()

Summary: `setValue()` incorrectly leaves `result` unfulfilled, since a default-constructed `Try` doesn't have either a value or an exception, so any futures constructed after the `SharedPromise` is fulfilled are never fulfilled.

Reviewed By: ispeters

Differential Revision: D49373014

fbshipit-source-id: 6cd890863c9a0e4c65eab71a3b24bb760f057f91

folly/experimental/coro/SharedPromise.h

@@ -163,7 +163,7 @@ void SharedPromise<T>::setValue(U&& input) {
 template <typename T>
 template <typename U, typename>
 void SharedPromise<T>::setValue() {
-  setTry({});
+  setTry(TryType{unit});
 }
 
 template <typename T>

folly/experimental/coro/test/SharedPromiseTest.cpp

@@ -289,11 +289,19 @@ TYPED_TEST(SharedPromiseTest, NoHeapAllocation) {
 }
 
 TEST(SharedPromiseTest, BasicVoid) {
-  auto promise = SharedPromise<void>{};
-  auto future = promise.getFuture();
+  {
+    auto promise = SharedPromise<void>{};
+    auto future = promise.getFuture();
+
+    promise.setValue();
+    blocking_wait(std::move(future));
+  }
 
-  promise.setValue();
-  blocking_wait(std::move(future));
+  {
+    auto promise = SharedPromise<void>{};
+    promise.setValue();
+    blocking_wait(promise.getFuture());
+  }
 }
 
 #endif