Add coroutine header fallback for C++20

include/cppcoro/static_thread_pool.hpp

@@ -11,7 +11,16 @@
 #include <thread>
 #include <vector>
 #include <mutex>
-#include <experimental/coroutine>
+
+#if __has_include(<coroutine>)
+	#include <coroutine>
+	namespace cppcoro_coroutine = std;
+#elif __has_include(<experimental/coroutine>)
+	#include <experimental/coroutine>
+	namespace cppcoro_coroutine = std::experimental;
+#else
+	#error "No coroutine header available"
+#endif
 
 namespace cppcoro
 {
@@ -38,15 +47,15 @@ namespace cppcoro
 			schedule_operation(static_thread_pool* tp) noexcept : m_threadPool(tp) {}
 
 			bool await_ready() noexcept { return false; }
-			void await_suspend(std::experimental::coroutine_handle<> awaitingCoroutine) noexcept;
+			void await_suspend(cppcoro_coroutine::coroutine_handle<> awaitingCoroutine) noexcept;
 			void await_resume() noexcept {}
 
 		private:
 
 			friend class static_thread_pool;
 
 			static_thread_pool* m_threadPool;
-			std::experimental::coroutine_handle<> m_awaitingCoroutine;
+			cppcoro_coroutine::coroutine_handle<> m_awaitingCoroutine;
 			schedule_operation* m_next;
 
 		};

include/cppcoro/task.hpp

@@ -49,9 +49,7 @@ namespace cppcoro
 				// were crashing under x86 optimised builds.
 				template<typename PROMISE>
 				CPPCORO_NOINLINE
-				void await_suspend(std::experimental::coroutine_handle<PROMISE> coroutine)
-				{
-					task_promise_base& promise = coroutine.promise();
+			void await_suspend(std::experimental::coroutine_handle<PROMISE> coroutine) noexcept
 
 					// Use 'release' memory semantics in case we finish before the
 					// awaiter can suspend so that the awaiting thread sees our

lib/static_thread_pool.cpp

@@ -319,7 +319,7 @@ namespace cppcoro
 	};
 
 	void static_thread_pool::schedule_operation::await_suspend(
-		std::experimental::coroutine_handle<> awaitingCoroutine) noexcept
+		cppcoro_coroutine::coroutine_handle<> awaitingCoroutine) noexcept
 	{
 		m_awaitingCoroutine = awaitingCoroutine;
 		m_threadPool->schedule_impl(this);