Make inline_executor the default candidate for associated_executor.

doc/requirements/asynchronous_operations.qbk

@@ -222,7 +222,7 @@ Certain objects that participate in asynchronous operations have an
 An asynchronous operation has an associated executor satisfying the [link
 boost_asio.reference.Executor1 `Executor`] requirements. If not otherwise specified by
 the asynchronous operation, this associated executor is an object of type
-`system_executor`.
+`inline_executor`.
 
 All asynchronous operations in this library have an associated I/O executor
 object that is determined as follows:

include/boost/asio/associated_executor.hpp

@@ -20,8 +20,9 @@
 #include <boost/asio/detail/functional.hpp>
 #include <boost/asio/detail/type_traits.hpp>
 #include <boost/asio/execution/executor.hpp>
+#include <boost/asio/execution_context.hpp>
+#include <boost/asio/inline_executor.hpp>
 #include <boost/asio/is_executor.hpp>
-#include <boost/asio/system_executor.hpp>
 
 #include <boost/asio/detail/push_options.hpp>
 
@@ -114,7 +115,7 @@ struct associated_executor_impl<T, E,
  * get(t,e) and with return type @c type or a (possibly const) reference to @c
  * type.
  */
-template <typename T, typename Executor = system_executor>
+template <typename T, typename Executor = inline_executor>
 struct associated_executor
 #if !defined(GENERATING_DOCUMENTATION)
   : detail::associated_executor_impl<T, Executor>
@@ -177,7 +178,7 @@ get_associated_executor(const T& t, ExecutionContext& ctx,
     typename ExecutionContext::executor_type>::get(t, ctx.get_executor());
 }
 
-template <typename T, typename Executor = system_executor>
+template <typename T, typename Executor = inline_executor>
 using associated_executor_t = typename associated_executor<T, Executor>::type;
 
 namespace detail {