Add makeNowTask / makeErrorNowTask

Summary:
Analog of `folly::coro::makeTask`.

The typical use-case is a "coroutine wrapper", i.e. a non-coro function that returns a trivial coroutine on some branches, and a potentially-suspending coro on other branches. The typical motivation for such wrappers (over just doing `co_return co_await subTask()`) is to guarantee that there's no coro frame overhead for the wrapper.

Reviewed By: ispeters

Differential Revision: D77400395

fbshipit-source-id: 5a2e5e81c439faf1f9f4d9d1d9dc02907d9b34c3

Author: snarkmaster

third-party/folly/src/folly/coro/AsyncGenerator.h

@@ -695,9 +695,8 @@ class AsyncGeneratorPromise final
     state_ = State::DONE;
   }
 
-  // FIXME: Much of this class is currenrly copy-pasted from `TaskPromiseBase`,
-  // and should be refactored to be use that, so as to avoid divergent behavior
-  // of `co_await`.
+  // FIXME: Much of this class is currently copy-pasted from `TaskPromiseBase`,
+  // Refactor this to use that, so as to avoid `co_await` behavior divergence.
 
   template <
       typename Awaitable,

third-party/folly/src/folly/coro/safe/NowTask.h

@@ -97,6 +97,9 @@ using NowTaskBase =
 template <safe_alias, typename>
 class SafeTask;
 
+template <safe_alias S, typename U>
+auto toNowTask(SafeTask<S, U>);
+
 template <typename T>
 class FOLLY_CORO_TASK_ATTRS NowTask final : public detail::NowTaskBase<T> {
  protected:
@@ -120,6 +123,33 @@ auto toNowTask(NowTask<T> t) {
   return NowTask<T>{std::move(t).unwrapTask()};
 }
 
+// Apparently, Clang 15 has a bug in prvalue semantics support, so it cannot
+// return immovable coroutines.
+#if !defined(__clang__) || __clang_major__ > 15
+
+/// Make a `NowTask` that trivially returns a value.
+template <class T>
+NowTask<T> makeNowTask(T t) {
+  co_return t;
+}
+
+/// Make a `NowTask` that trivially returns no value
+inline NowTask<> makeNowTask() {
+  co_return;
+}
+/// Same as makeNowTask(). See Unit
+inline NowTask<> makeNowTask(Unit) {
+  co_return;
+}
+
+/// Make a `NowTask` that will trivially yield an exception.
+template <class T>
+NowTask<T> makeErrorNowTask(exception_wrapper ew) {
+  co_yield co_error(std::move(ew));
+}
+
+#endif // no `makeNowTask` on old/buggy clang
+
 } // namespace folly::coro
 
 template <typename T>