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diff --git a/lib/asio/associated_executor.hpp b/lib/asio/associated_executor.hpp
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+//
+// associated_executor.hpp
+// ~~~~~~~~~~~~~~~~~~~~~~~
+//
+// Copyright (c) 2003-2018 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 ASIO_ASSOCIATED_EXECUTOR_HPP
+#define ASIO_ASSOCIATED_EXECUTOR_HPP
+
+#if defined(_MSC_VER) && (_MSC_VER >= 1200)
+# pragma once
+#endif // defined(_MSC_VER) && (_MSC_VER >= 1200)
+
+#include "asio/detail/config.hpp"
+#include "asio/detail/type_traits.hpp"
+#include "asio/is_executor.hpp"
+#include "asio/system_executor.hpp"
+
+#include "asio/detail/push_options.hpp"
+
+namespace asio {
+namespace detail {
+
+template <typename>
+struct associated_executor_check
+{
+  typedef void type;
+};
+
+template <typename T, typename E, typename = void>
+struct associated_executor_impl
+{
+  typedef E type;
+
+  static type get(const T&, const E& e) ASIO_NOEXCEPT
+  {
+    return e;
+  }
+};
+
+template <typename T, typename E>
+struct associated_executor_impl<T, E,
+  typename associated_executor_check<typename T::executor_type>::type>
+{
+  typedef typename T::executor_type type;
+
+  static type get(const T& t, const E&) ASIO_NOEXCEPT
+  {
+    return t.get_executor();
+  }
+};
+
+} // namespace detail
+
+/// Traits type used to obtain the executor associated with an object.
+/**
+ * A program may specialise this traits type if the @c T template parameter in
+ * the specialisation is a user-defined type. The template parameter @c
+ * Executor shall be a type meeting the Executor requirements.
+ *
+ * Specialisations shall meet the following requirements, where @c t is a const
+ * reference to an object of type @c T, and @c e is an object of type @c
+ * Executor.
+ *
+ * @li Provide a nested typedef @c type that identifies a type meeting the
+ * Executor requirements.
+ *
+ * @li Provide a noexcept static member function named @c get, callable as @c
+ * get(t) and with return type @c type.
+ *
+ * @li Provide a noexcept static member function named @c get, callable as @c
+ * get(t,e) and with return type @c type.
+ */
+template <typename T, typename Executor = system_executor>
+struct associated_executor
+{
+  /// If @c T has a nested type @c executor_type, <tt>T::executor_type</tt>.
+  /// Otherwise @c Executor.
+#if defined(GENERATING_DOCUMENTATION)
+  typedef see_below type;
+#else // defined(GENERATING_DOCUMENTATION)
+  typedef typename detail::associated_executor_impl<T, Executor>::type type;
+#endif // defined(GENERATING_DOCUMENTATION)
+
+  /// If @c T has a nested type @c executor_type, returns
+  /// <tt>t.get_executor()</tt>. Otherwise returns @c ex.
+  static type get(const T& t,
+      const Executor& ex = Executor()) ASIO_NOEXCEPT
+  {
+    return detail::associated_executor_impl<T, Executor>::get(t, ex);
+  }
+};
+
+/// Helper function to obtain an object's associated executor.
+/**
+ * @returns <tt>associated_executor<T>::get(t)</tt>
+ */
+template <typename T>
+inline typename associated_executor<T>::type
+get_associated_executor(const T& t) ASIO_NOEXCEPT
+{
+  return associated_executor<T>::get(t);
+}
+
+/// Helper function to obtain an object's associated executor.
+/**
+ * @returns <tt>associated_executor<T, Executor>::get(t, ex)</tt>
+ */
+template <typename T, typename Executor>
+inline typename associated_executor<T, Executor>::type
+get_associated_executor(const T& t, const Executor& ex,
+    typename enable_if<is_executor<
+      Executor>::value>::type* = 0) ASIO_NOEXCEPT
+{
+  return associated_executor<T, Executor>::get(t, ex);
+}
+
+/// Helper function to obtain an object's associated executor.
+/**
+ * @returns <tt>associated_executor<T, typename
+ * ExecutionContext::executor_type>::get(t, ctx.get_executor())</tt>
+ */
+template <typename T, typename ExecutionContext>
+inline typename associated_executor<T,
+  typename ExecutionContext::executor_type>::type
+get_associated_executor(const T& t, ExecutionContext& ctx,
+    typename enable_if<is_convertible<ExecutionContext&,
+      execution_context&>::value>::type* = 0) ASIO_NOEXCEPT
+{
+  return associated_executor<T,
+    typename ExecutionContext::executor_type>::get(t, ctx.get_executor());
+}
+
+#if defined(ASIO_HAS_ALIAS_TEMPLATES)
+
+template <typename T, typename Executor = system_executor>
+using associated_executor_t = typename associated_executor<T, Executor>::type;
+
+#endif // defined(ASIO_HAS_ALIAS_TEMPLATES)
+
+} // namespace asio
+
+#include "asio/detail/pop_options.hpp"
+
+#endif // ASIO_ASSOCIATED_EXECUTOR_HPP