妈妈，我看不懂 is_instance_of_v 背后的魔法了

include/ucoro/awaitable.hpp

@@ -73,18 +73,33 @@ namespace ucoro
 	template<typename T>
 	struct local_storage_t
 	{
-		typedef T value_type;
-		typedef void local_storage_type_detect_tag;
 	};
 
 	inline constexpr local_storage_t<void> local_storage;
 
-	template <typename T>
-	concept is_a_local_storage_t = std::is_same_v<typename T::local_storage_type_detect_tag, typename T::local_storage_type_detect_tag>;
-
 	//////////////////////////////////////////////////////////////////////////
 	namespace detail
 	{
+
+		// 用于判定 T 是否是一个 U<anytype> 的类型
+		// 比如
+		// is_instance_of_v<std::vector<int>,std::vector>;  // true
+		// is_instance_of_v<std::vector<int>,std::list>;  // false
+		template<class T, template<class...> class U>
+		inline constexpr bool is_instance_of_v = std::false_type{};
+
+		template<template<class...> class U, class... Vs>
+		inline constexpr bool is_instance_of_v<U<Vs...>,U> = std::true_type{};
+
+		template<class LocalStorage>
+		struct local_storage_value_type;
+
+		template<class ValueType>
+		struct local_storage_value_type<local_storage_t<ValueType>>
+		{
+			typedef ValueType value_type;
+		};
+
 		template<typename T>
 		concept is_valid_await_suspend_return_value =
 			std::convertible_to<T, std::coroutine_handle<>> || std::is_void_v<T> || std::is_same_v<T, bool>;
@@ -106,11 +121,7 @@ namespace ucoro
 		template<typename T>
 		concept is_awaitable_v = is_awaiter_v<T> || has_operator_co_await<T>;
 
-		// 用于判定 T 是否是一个 awaitable<>::promise_type 的类型, 即: 拥有 local_ 成员。
-		template<typename T>
-		concept is_awaitable_promise_type_v = requires (T a) {
-			{ a.local_ } -> std::convertible_to<std::shared_ptr<std::any>>;
-		};
+
 	} // namespace detail
 
 	struct debug_coro_promise
@@ -266,16 +277,24 @@ namespace ucoro
 		template<typename A>
 		auto await_transform(A&& awaiter) const
 		{
-			if constexpr ( is_a_local_storage_t<std::decay_t<A>> )
+			if constexpr (detail::is_instance_of_v<std::decay_t<A>, local_storage_t>)
 			{
-				return local_storage_awaiter<typename std::decay_t<A>::value_type>{this};
+				// 类型 A 是 local_storage_t<> 的一种
+				return local_storage_awaiter<typename detail::local_storage_value_type<std::decay_t<A>>::value_type>{this};
+			}
+			else if constexpr (detail::is_instance_of_v<std::decay_t<A>, awaitable>)
+			{
+				// 类型 A 是 awaitable<> 的一种
+				static_assert(std::is_rvalue_reference_v<decltype(awaiter)>, "co_await must be used on rvalue");
+				return std::forward<A>(awaiter);
 			}
-			else if constexpr ( detail::is_awaitable_v<std::decay_t<A>> )
+			else if constexpr (detail::is_awaitable_v<std::decay_t<A>>)
 			{
+				// 类型 A 有 三件套
 				static_assert(std::is_rvalue_reference_v<decltype(awaiter)>, "co_await must be used on rvalue");
 				return std::forward<A>(awaiter);
 			}
-			else if constexpr ( requires (A ) { await_transformer<A>::await_transform; })
+			else if constexpr (requires (A) { await_transformer<A>::await_transform; })
 			{
 				return await_transformer<A>::await_transform(std::move(awaiter));
 			}
@@ -397,7 +416,8 @@ namespace ucoro
 		template<typename PromiseType>
 		auto await_suspend(std::coroutine_handle<PromiseType> continuation)
 		{
-			if constexpr (detail::is_awaitable_promise_type_v<PromiseType>)
+			// PromiseType 是 awaitable<any>::promise_type 的一种
+			if constexpr (detail::is_instance_of_v<PromiseType, awaitable_promise>)
 			{
 				auto& calee_promise = this_->current_coro_handle_.promise();
 				auto& caller_promise = continuation.promise();

tests/test3/test.cpp

@@ -7,7 +7,7 @@ int main(int argc, char **argv)
 	using CallbackAwaiterType0 = ucoro::CallbackAwaiter<void, decltype([](auto h) {}) >;
 	using CallbackAwaiterType1 = ucoro::CallbackAwaiter<int, decltype([](auto h) {}) > ;
 
-	static_assert(ucoro::is_a_local_storage_t<ucoro::local_storage_t<void>>, "not a local_storage_t");
+	static_assert(ucoro::detail::is_instance_of_v<ucoro::local_storage_t<void>, ucoro::local_storage_t>, "not a local_storage_t");
 
 	static_assert(ucoro::detail::is_awaiter_v < CallbackAwaiterType0 >, "not a coroutine");
 	static_assert(ucoro::detail::is_awaiter_v < CallbackAwaiterType1 >, "not a coroutine");