使用 is_awaiter_v ， 兼容下 test3

include/ucoro/awaitable.hpp

@@ -51,15 +51,25 @@ namespace ucoro
 			: std::true_type
 		{};
 
+		// NOTE: We're accepting a return value of coroutine_handle<P> here
+		// which is an extension supported by Clang which is not yet part of
+		// the C++ coroutines TS.
+		template<typename T>
+		struct is_valid_await_suspend_return_value : std::disjunction<
+			std::is_void<T>,
+			std::is_same<T, bool>,
+			is_coroutine_handle<T>>
+		{};
+
 		// NOTE: We're testing whether await_suspend() will be callable using an
 		// arbitrary coroutine_handle here by checking if it supports being passed
 		// a coroutine_handle<void>. This may result in a false-result for some
 		// types which are only awaitable within a certain context.
 		template <typename T>
-		concept is_awaiter = requires ( T a)
+		concept is_awaiter_v = requires ( T a)
 		{
 			{ a.await_ready() } -> std::convertible_to<bool>;
-			{ a.await_suspend(std::declval<std::coroutine_handle<>>()) }; //-> std::convertible_to<std::coroutine_handle<>>;
+			{ a.await_suspend(std::declval<std::coroutine_handle<>>()) };// -> ( std::convertible_to<std::coroutine_handle<>> | std::convertible_to<bool> );
 			{ a.await_resume() };
 		};
 	} // namespace detail
@@ -149,7 +159,7 @@ namespace ucoro
 
 		void unhandled_exception() {}
 
-		template <typename A> requires( detail::is_awaiter<A> )
+		template <typename A> requires( detail::is_awaiter_v<A> )
 		auto await_transform(A awaiter) const
 		{
 			return awaiter;