Add AwaitResult metafunction

rt/include/typon/meta.hpp

@@ -21,13 +21,19 @@ namespace typon::meta
   };
 
 
+  /* inspired by and simplified from
+     https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1288r0.pdf
+   */
+
   template <typename T>
-  concept Awaitable = Awaiter<std::remove_cvref_t<T>> || requires (T && a)
+  concept HasCoAwait = requires (T && a)
   {
     // simplified
     std::forward<T>(a).operator co_await();
   };
 
+  template <typename T>
+  concept Awaitable = Awaiter<T> || HasCoAwait<T>;
 
   template <typename T>
   requires (! Awaitable<T>)
@@ -46,6 +52,25 @@ namespace typon::meta
     }
   };
 
+
+  template <typename T>
+  struct AwaitResult_t {
+    using type = T;
+  };
+
+  template <typename T>
+  using AwaitResult = typename AwaitResult_t<T>::type;
+
+  template <Awaiter T>
+  struct AwaitResult_t<T> {
+    using type = decltype(std::declval<T>().await_resume());
+  };
+
+  template <HasCoAwait T>
+  struct AwaitResult_t<T> {
+    using type = AwaitResult<decltype(std::declval<T>().operator co_await())>;
+  };
+
 }