Add associator trait.

The associator trait is used to generically forward associators, such as
associated_executor<> and associated_allocator<>, through intermediate
completion handlers. For example:

    template <typename Handler>
    struct intermediate_handler
    {
      Handler handler_;

      template <typename... Args>
      void operator()(Args&... args)
      {
        // ...
      }
    };

    namespace boost {
    namespace asio {

      template <
          template <typename, typename> class Associator,
          typename Handler,
          typename DefaultCandidate>
      struct associator<
          Associator,
          intermediate_handler<Handler>,
          DefaultCandidate>
      {
        using type =
          typename Associator<Handler, DefaultCandidate>::type;

        static type get(
            const intermediate_handler<Handler>& h,
            const DefaultCandidate& c = DefaultCandidate()) noexcept
        {
          return Associator<Handler, DefaultCandidate>::get(
              h.handler_, c);
        }
      };

    } // namespace asio
    } // namespace boost

include/boost/asio.hpp

@@ -19,6 +19,7 @@
 
 #include <boost/asio/associated_allocator.hpp>
 #include <boost/asio/associated_executor.hpp>
+#include <boost/asio/associator.hpp>
 #include <boost/asio/async_result.hpp>
 #include <boost/asio/awaitable.hpp>
 #include <boost/asio/basic_datagram_socket.hpp>

include/boost/asio/associated_allocator.hpp

@@ -17,6 +17,7 @@
 
 #include <boost/asio/detail/config.hpp>
 #include <memory>
+#include <boost/asio/associator.hpp>
 #include <boost/asio/detail/functional.hpp>
 #include <boost/asio/detail/type_traits.hpp>
 
@@ -24,9 +25,25 @@
 
 namespace boost {
 namespace asio {
+
+template <typename T, typename Allocator>
+struct associated_allocator;
+
 namespace detail {
 
-template <typename T, typename E, typename = void>
+template <typename T, typename = void>
+struct has_allocator_type : false_type
+{
+};
+
+template <typename T>
+struct has_allocator_type<T,
+  typename void_type<typename T::executor_type>::type>
+    : true_type
+{
+};
+
+template <typename T, typename E, typename = void, typename = void>
 struct associated_allocator_impl
 {
   typedef E type;
@@ -49,6 +66,17 @@ struct associated_allocator_impl<T, E,
   }
 };
 
+template <typename T, typename E>
+struct associated_allocator_impl<T, E,
+  typename enable_if<
+    !has_allocator_type<T>::value
+  >::type,
+  typename void_type<
+    typename associator<associated_allocator, T, E>::type
+  >::type> : associator<associated_allocator, T, E>
+{
+};
+
 } // namespace detail
 
 /// Traits type used to obtain the allocator associated with an object.

include/boost/asio/associated_executor.hpp

@@ -16,6 +16,7 @@
 #endif // defined(_MSC_VER) && (_MSC_VER >= 1200)
 
 #include <boost/asio/detail/config.hpp>
+#include <boost/asio/associator.hpp>
 #include <boost/asio/detail/functional.hpp>
 #include <boost/asio/detail/type_traits.hpp>
 #include <boost/asio/execution/executor.hpp>
@@ -26,9 +27,25 @@
 
 namespace boost {
 namespace asio {
+
+template <typename T, typename Executor>
+struct associated_executor;
+
 namespace detail {
 
-template <typename T, typename E, typename = void>
+template <typename T, typename = void>
+struct has_executor_type : false_type
+{
+};
+
+template <typename T>
+struct has_executor_type<T,
+  typename void_type<typename T::executor_type>::type>
+    : true_type
+{
+};
+
+template <typename T, typename E, typename = void, typename = void>
 struct associated_executor_impl
 {
   typedef void asio_associated_executor_is_unspecialised;
@@ -53,6 +70,17 @@ struct associated_executor_impl<T, E,
   }
 };
 
+template <typename T, typename E>
+struct associated_executor_impl<T, E,
+  typename enable_if<
+    !has_executor_type<T>::value
+  >::type,
+  typename void_type<
+    typename associator<associated_executor, T, E>::type
+  >::type> : associator<associated_executor, T, E>
+{
+};
+
 } // namespace detail
 
 /// Traits type used to obtain the executor associated with an object.

include/boost/asio/associator.hpp

@@ -0,0 +1,37 @@
+//
+// associator.hpp
+// ~~~~~~~~~~~~~~
+//
+// Copyright (c) 2003-2021 Christopher M. Kohlhoff (chris at kohlhoff dot com)
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+
+#ifndef BOOST_ASIO_ASSOCIATOR_HPP
+#define BOOST_ASIO_ASSOCIATOR_HPP
+
+#if defined(_MSC_VER) && (_MSC_VER >= 1200)
+# pragma once
+#endif // defined(_MSC_VER) && (_MSC_VER >= 1200)
+
+#include <boost/asio/detail/config.hpp>
+
+#include <boost/asio/detail/push_options.hpp>
+
+namespace boost {
+namespace asio {
+
+/// Used to generically specialise associators for a type.
+template <template <typename, typename> class Associator,
+    typename T, typename DefaultCandidate>
+struct associator
+{
+};
+
+} // namespace asio
+} // namespace boost
+
+#include <boost/asio/detail/pop_options.hpp>
+
+#endif // BOOST_ASIO_ASSOCIATOR_HPP