Conditionally define immediate_executor_type in async_base

Fixes #3002

include/boost/beast/core/async_base.hpp

@@ -177,7 +177,8 @@ template<
 >
 class async_base
 #if ! BOOST_BEAST_DOXYGEN
-    : private boost::empty_value<Allocator>
+    : public detail::with_immediate_executor_type<Handler>
+    , private boost::empty_value<Allocator>
 #endif
 {
     static_assert(
@@ -193,7 +194,7 @@ public:
     If a class derived from @ref boost::beast::async_base is a completion
     handler, then the associated executor of the derived class will
     be this type.
-*/
+    */
     using executor_type =
 #if BOOST_BEAST_DOXYGEN
         __implementation_defined__;
@@ -210,19 +211,12 @@ public:
     If a class derived from @ref boost::beast::async_base is a completion
     handler, then the associated immediage executor of the derived class will
     be this type.
-*/
-    using immediate_executor_type =
+    */
 #if BOOST_BEAST_DOXYGEN
+    using immediate_executor_type =
         __implementation_defined__;
-#else
-        typename
-        net::associated_immediate_executor<
-            Handler,
-            typename detail::select_work_guard_t<Executor1>::executor_type
-                >::type;
 #endif
 
-
   private:
 
     virtual
@@ -333,8 +327,10 @@ public:
         handler, then the object returned from this function will be used
         as the associated immediate executor of the derived class.
     */
-    immediate_executor_type
+    auto
     get_immediate_executor() const noexcept
+        -> decltype(net::get_associated_immediate_executor(
+            h_, wg1_.get_executor()))
     {
         return net::get_associated_immediate_executor(
             h_, wg1_.get_executor());

include/boost/beast/core/detail/async_base.hpp

@@ -10,7 +10,9 @@
 #ifndef BOOST_BEAST_CORE_DETAIL_ASYNC_BASE_HPP
 #define BOOST_BEAST_CORE_DETAIL_ASYNC_BASE_HPP
 
-#include <boost/core/exchange.hpp>
+#include <boost/beast/core/detail/type_traits.hpp>
+
+#include <boost/asio/associated_immediate_executor.hpp>
 
 namespace boost {
 namespace beast {
@@ -39,6 +41,18 @@ protected:
     virtual void destroy() = 0;
 };
 
+template<typename Handler, typename = void>
+struct with_immediate_executor_type
+{
+};
+
+template<typename Handler>
+struct with_immediate_executor_type<Handler,
+    void_t<typename Handler::immediate_executor_type>>
+{
+    using immediate_executor_type = typename Handler::immediate_executor_type;
+};
+
 } // detail
 } // beast
 } // boost