Add awaitable to forward declarations

include/asio/awaitable.hpp

@@ -47,11 +47,20 @@ template <typename, typename> class awaitable_frame;
 
 } // namespace detail
 
+#if !defined(ASIO_AWAITABLE_FWD_DECL)
+#define ASIO_AWAITABLE_FWD_DECL
+
+// Forward declaration with defaulted arguments.
+template <typename T, typename Executor = any_io_executor>
+class awaitable;
+
+#endif // !defined(ASIO_AWAITABLE_FWD_DECL)
+
 /// The return type of a coroutine or asynchronous operation.
 /**
  * @sa @ref overview_cpp20_coroutines "C++20 coroutines support"
  */
-template <typename T, typename Executor = any_io_executor>
+template <typename T, typename Executor>
 class ASIO_NODISCARD awaitable
 {
 public:

include/asio/fwd.hpp

@@ -82,6 +82,19 @@ class any_completion_executor;
 
 #endif // defined(ASIO_USE_TS_EXECUTOR_AS_DEFAULT)
 
+#if defined(ASIO_HAS_CO_AWAIT)
+
+#if !defined(ASIO_AWAITABLE_FWD_DECL)
+#define ASIO_AWAITABLE_FWD_DECL
+
+// Forward declaration with defaulted arguments.
+template <typename T, typename Executor = any_io_executor>
+class awaitable;
+
+#endif // !defined(ASIO_AWAITABLE_FWD_DECL)
+
+#endif // defined(ASIO_HAS_CO_AWAIT)
+
 template <typename... Signatures>
 class any_completion_handler;
 

src/tests/unit/fwd_first.cpp

@@ -16,6 +16,10 @@
 // Test that header file is self-contained.
 #include "asio/fwd.hpp"
 
+#if defined(ASIO_HAS_CO_AWAIT)
+asio::awaitable<void> fwd_awaitable();
+#endif // defined(ASIO_HAS_CO_AWAIT)
+
 // Test that forward declarations don't conflict with full declarations.
 #include "asio.hpp"