feat: use conditional coroutine support in public API

This is to make the public API buildable also under C++17.

Author: sangelovic

include/sdbus-c++/Awaitable.h

@@ -29,7 +29,9 @@
 #define SDBUS_CXX_AWAITABLE_H_
 
 #include <atomic>
+#if __has_include(<coroutine>)
 #include <coroutine>
+#endif
 #include <cstdint>
 #include <exception>
 #include <memory>
@@ -61,8 +63,17 @@ struct AwaitableData
 {
     using result_type = std::conditional_t<std::is_void_v<T>, std::monostate, T>;
     std::variant<result_type, std::exception_ptr> result;
+#ifdef __cpp_lib_coroutine
     std::coroutine_handle<> handle;
+#endif // __cpp_lib_coroutine
     std::atomic<AwaitableState> status{AwaitableState::NotReady};
+
+    void resumeCoroutine()
+    {
+#ifdef __cpp_lib_coroutine
+        handle.resume();
+#endif // __cpp_lib_coroutine
+    }
 };
 
 /********************************************//**
@@ -82,6 +93,9 @@ struct AwaitableData
  * instance, such as IProxy::callMethodAsync with with_awaitable_t tag,
  * or the .getResultAsAwaitable() methods of the high-level API.
  *
+ * The class represents nothing, i.e. is a simple placeholder class, if the API
+ * is used as C++17 or with a standard library not supporting coroutines.
+ *
  ***********************************************/
 template <typename T>
 class Awaitable
@@ -92,6 +106,8 @@ class Awaitable
     {
     }
 
+#ifdef __cpp_lib_coroutine
+
     // Called when the coroutine is co_await'ed. Returns true if the coroutine should be suspended.
     [[nodiscard]] bool await_ready() const noexcept
     {
@@ -121,6 +137,8 @@ class Awaitable
             return std::get<T>(std::move(data_->result));
     }
 
+#endif // __cpp_lib_coroutine
+
 private:
     std::shared_ptr<AwaitableData<T>> data_;
 };

include/sdbus-c++/ConvenienceApiClasses.inl

@@ -374,7 +374,7 @@ namespace sdbus {
 
             auto previous = data->status.exchange(AwaitableState::Completed, std::memory_order_acq_rel);
             if (previous == AwaitableState::Waiting)
-                data->handle.resume();
+                data->resumeCoroutine();
         });
 
         return Awaitable(data);

src/Proxy.cpp

@@ -199,7 +199,7 @@ Awaitable<MethodReply> Proxy::callMethodAsync(const MethodCall& message, uint64_
 
         auto previous = data->status.exchange(AwaitableState::Completed, std::memory_order_acq_rel);
         if (previous == AwaitableState::Waiting)
-            data->handle.resume();
+            data->resumeCoroutine();
     };
 
     (void)Proxy::callMethodAsync(message, std::move(asyncReplyCallback), timeout);