diff --git a/asio/include/asio/execution/blocking.hpp b/asio/include/asio/execution/blocking.hpp
index 2f2c3991..ebfa5c13 100644
--- a/asio/include/asio/execution/blocking.hpp
+++ b/asio/include/asio/execution/blocking.hpp
@@ -270,8 +270,12 @@ struct blocking_t
   static_query(
       typename enable_if<
         !traits::query_static_constexpr_member<T, blocking_t>::is_valid
-          && !traits::query_member<T, blocking_t>::is_valid
-          && traits::static_query<T, possibly_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        !traits::query_member<T, blocking_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        traits::static_query<T, possibly_t>::is_valid
       >::type* = 0) ASIO_NOEXCEPT
   {
     return traits::static_query<T, possibly_t>::value();
@@ -283,9 +287,15 @@ struct blocking_t
   static_query(
       typename enable_if<
         !traits::query_static_constexpr_member<T, blocking_t>::is_valid
-          && !traits::query_member<T, blocking_t>::is_valid
-          && !traits::static_query<T, possibly_t>::is_valid
-          && traits::static_query<T, always_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        !traits::query_member<T, blocking_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        !traits::static_query<T, possibly_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        traits::static_query<T, always_t>::is_valid
       >::type* = 0) ASIO_NOEXCEPT
   {
     return traits::static_query<T, always_t>::value();
@@ -297,10 +307,18 @@ struct blocking_t
   static_query(
       typename enable_if<
         !traits::query_static_constexpr_member<T, blocking_t>::is_valid
-          && !traits::query_member<T, blocking_t>::is_valid
-          && !traits::static_query<T, possibly_t>::is_valid
-          && !traits::static_query<T, always_t>::is_valid
-          && traits::static_query<T, never_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        !traits::query_member<T, blocking_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        !traits::static_query<T, possibly_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        !traits::static_query<T, always_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        traits::static_query<T, never_t>::is_valid
       >::type* = 0) ASIO_NOEXCEPT
   {
     return traits::static_query<T, never_t>::value();
@@ -353,7 +371,9 @@ struct blocking_t
       const Executor& ex, convertible_from_blocking_t,
       typename enable_if<
         !can_query<const Executor&, possibly_t>::value
-          && can_query<const Executor&, always_t>::value
+      >::type* = 0,
+      typename enable_if<
+        can_query<const Executor&, always_t>::value
       >::type* = 0)
 #if !defined(__clang__) // Clang crashes if noexcept is used here.
 #if defined(ASIO_MSVC) // Visual C++ wants the type to be qualified.
@@ -373,8 +393,12 @@ struct blocking_t
       const Executor& ex, convertible_from_blocking_t,
       typename enable_if<
         !can_query<const Executor&, possibly_t>::value
-          && !can_query<const Executor&, always_t>::value
-          && can_query<const Executor&, never_t>::value
+      >::type* = 0,
+      typename enable_if<
+        !can_query<const Executor&, always_t>::value
+      >::type* = 0,
+      typename enable_if<
+        can_query<const Executor&, never_t>::value
       >::type* = 0)
 #if !defined(__clang__) // Clang crashes if noexcept is used here.
 #if defined(ASIO_MSVC) // Visual C++ wants the type to be qualified.
@@ -468,10 +492,18 @@ struct possibly_t
   static ASIO_CONSTEXPR possibly_t static_query(
       typename enable_if<
         !traits::query_static_constexpr_member<T, possibly_t>::is_valid
-          && !traits::query_member<T, possibly_t>::is_valid
-          && !traits::query_free<T, possibly_t>::is_valid
-          && !can_query<T, always_t<I> >::value
-          && !can_query<T, never_t<I> >::value
+      >::type* = 0,
+      typename enable_if<
+        !traits::query_member<T, possibly_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        !traits::query_free<T, possibly_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        !can_query<T, always_t<I> >::value
+      >::type* = 0,
+      typename enable_if<
+        !can_query<T, never_t<I> >::value
       >::type* = 0) ASIO_NOEXCEPT
   {
     return possibly_t();
@@ -755,7 +787,9 @@ struct always_t
       const Executor& e, const always_t&,
       typename enable_if<
         is_executor<Executor>::value
-        && traits::static_require<
+      >::type* = 0,
+      typename enable_if<
+        traits::static_require<
           const Executor&,
           blocking_adaptation::allowed_t<0>
         >::is_valid
diff --git a/asio/include/asio/execution/blocking_adaptation.hpp b/asio/include/asio/execution/blocking_adaptation.hpp
index 49d364d7..fee7717e 100644
--- a/asio/include/asio/execution/blocking_adaptation.hpp
+++ b/asio/include/asio/execution/blocking_adaptation.hpp
@@ -225,8 +225,12 @@ struct blocking_adaptation_t
       typename enable_if<
         !traits::query_static_constexpr_member<
             T, blocking_adaptation_t>::is_valid
-          && !traits::query_member<T, blocking_adaptation_t>::is_valid
-          && traits::static_query<T, disallowed_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        !traits::query_member<T, blocking_adaptation_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        traits::static_query<T, disallowed_t>::is_valid
       >::type* = 0) ASIO_NOEXCEPT
   {
     return traits::static_query<T, disallowed_t>::value();
@@ -239,9 +243,15 @@ struct blocking_adaptation_t
       typename enable_if<
         !traits::query_static_constexpr_member<
             T, blocking_adaptation_t>::is_valid
-          && !traits::query_member<T, blocking_adaptation_t>::is_valid
-          && !traits::static_query<T, disallowed_t>::is_valid
-          && traits::static_query<T, allowed_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        !traits::query_member<T, blocking_adaptation_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        !traits::static_query<T, disallowed_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        traits::static_query<T, allowed_t>::is_valid
       >::type* = 0) ASIO_NOEXCEPT
   {
     return traits::static_query<T, allowed_t>::value();
@@ -299,7 +309,9 @@ struct blocking_adaptation_t
       const Executor& ex, convertible_from_blocking_adaptation_t,
       typename enable_if<
         !can_query<const Executor&, disallowed_t>::value
-          && can_query<const Executor&, allowed_t>::value
+      >::type* = 0,
+      typename enable_if<
+        can_query<const Executor&, allowed_t>::value
       >::type* = 0)
 #if !defined(__clang__) // Clang crashes if noexcept is used here.
 #if defined(ASIO_MSVC) // Visual C++ wants the type to be qualified.
@@ -392,9 +404,15 @@ struct disallowed_t
   static ASIO_CONSTEXPR disallowed_t static_query(
       typename enable_if<
         !traits::query_static_constexpr_member<T, disallowed_t>::is_valid
-          && !traits::query_member<T, disallowed_t>::is_valid
-          && !traits::query_free<T, disallowed_t>::is_valid
-          && !can_query<T, allowed_t<I> >::value
+      >::type* = 0,
+      typename enable_if<
+        !traits::query_member<T, disallowed_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        !traits::query_free<T, disallowed_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        !can_query<T, allowed_t<I> >::value
       >::type* = 0) ASIO_NOEXCEPT
   {
     return disallowed_t();
diff --git a/asio/include/asio/execution/bulk_execute.hpp b/asio/include/asio/execution/bulk_execute.hpp
index 001b6088..d3a08dd8 100644
--- a/asio/include/asio/execution/bulk_execute.hpp
+++ b/asio/include/asio/execution/bulk_execute.hpp
@@ -124,7 +124,8 @@ enum overload_type
   ill_formed
 };
 
-template <typename S, typename Args, typename = void>
+template <typename S, typename Args, typename = void, typename = void,
+    typename = void, typename = void, typename = void, typename = void>
 struct call_traits
 {
   ASIO_STATIC_CONSTEXPR(overload_type, overload = ill_formed);
@@ -135,15 +136,15 @@ struct call_traits
 template <typename S, typename F, typename N>
 struct call_traits<S, void(F, N),
   typename enable_if<
-    (
-      is_convertible<N, std::size_t>::value
-      &&
-      bulk_execute_member<S, F, N>::is_valid
-      &&
-      is_sender<
-        typename bulk_execute_member<S, F, N>::result_type
-      >::value
-    )
+    is_convertible<N, std::size_t>::value
+  >::type,
+  typename enable_if<
+    bulk_execute_member<S, F, N>::is_valid
+  >::type,
+  typename enable_if<
+    is_sender<
+      typename bulk_execute_member<S, F, N>::result_type
+    >::value
   >::type> :
   bulk_execute_member<S, F, N>
 {
@@ -153,17 +154,18 @@ struct call_traits<S, void(F, N),
 template <typename S, typename F, typename N>
 struct call_traits<S, void(F, N),
   typename enable_if<
-    (
-      is_convertible<N, std::size_t>::value
-      &&
-      !bulk_execute_member<S, F, N>::is_valid
-      &&
-      bulk_execute_free<S, F, N>::is_valid
-      &&
-      is_sender<
-        typename bulk_execute_free<S, F, N>::result_type
-      >::value
-    )
+    is_convertible<N, std::size_t>::value
+  >::type,
+  typename enable_if<
+    !bulk_execute_member<S, F, N>::is_valid
+  >::type,
+  typename enable_if<
+    bulk_execute_free<S, F, N>::is_valid
+  >::type,
+  typename enable_if<
+    is_sender<
+      typename bulk_execute_free<S, F, N>::result_type
+    >::value
   >::type> :
   bulk_execute_free<S, F, N>
 {
@@ -173,26 +175,29 @@ struct call_traits<S, void(F, N),
 template <typename S, typename F, typename N>
 struct call_traits<S, void(F, N),
   typename enable_if<
-    (
-      is_convertible<N, std::size_t>::value
-      &&
-      !bulk_execute_member<S, F, N>::is_valid
-      &&
-      !bulk_execute_free<S, F, N>::is_valid
-      &&
-      is_sender<S>::value
-      &&
-      is_same<
-        typename result_of<
-          F(typename executor_index<typename remove_cvref<S>::type>::type)
-        >::type,
-        typename result_of<
-          F(typename executor_index<typename remove_cvref<S>::type>::type)
-        >::type
-      >::value
-      &&
-      static_require<S, bulk_guarantee_t::unsequenced_t>::is_valid
-    )
+    is_convertible<N, std::size_t>::value
+  >::type,
+  typename enable_if<
+    !bulk_execute_member<S, F, N>::is_valid
+  >::type,
+  typename enable_if<
+    !bulk_execute_free<S, F, N>::is_valid
+  >::type,
+  typename enable_if<
+    is_sender<S>::value
+  >::type,
+  typename enable_if<
+    is_same<
+      typename result_of<
+        F(typename executor_index<typename remove_cvref<S>::type>::type)
+      >::type,
+      typename result_of<
+        F(typename executor_index<typename remove_cvref<S>::type>::type)
+      >::type
+    >::value
+  >::type,
+  typename enable_if<
+    static_require<S, bulk_guarantee_t::unsequenced_t>::is_valid
   >::type>
 {
   ASIO_STATIC_CONSTEXPR(overload_type, overload = adapter);
diff --git a/asio/include/asio/execution/bulk_guarantee.hpp b/asio/include/asio/execution/bulk_guarantee.hpp
index 28b25854..158d350f 100644
--- a/asio/include/asio/execution/bulk_guarantee.hpp
+++ b/asio/include/asio/execution/bulk_guarantee.hpp
@@ -259,8 +259,12 @@ struct bulk_guarantee_t
   static_query(
       typename enable_if<
         !traits::query_static_constexpr_member<T, bulk_guarantee_t>::is_valid
-          && !traits::query_member<T, bulk_guarantee_t>::is_valid
-          && traits::static_query<T, unsequenced_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        !traits::query_member<T, bulk_guarantee_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        traits::static_query<T, unsequenced_t>::is_valid
       >::type* = 0) ASIO_NOEXCEPT
   {
     return traits::static_query<T, unsequenced_t>::value();
@@ -272,9 +276,15 @@ struct bulk_guarantee_t
   static_query(
       typename enable_if<
         !traits::query_static_constexpr_member<T, bulk_guarantee_t>::is_valid
-          && !traits::query_member<T, bulk_guarantee_t>::is_valid
-          && !traits::static_query<T, unsequenced_t>::is_valid
-          && traits::static_query<T, sequenced_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        !traits::query_member<T, bulk_guarantee_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        !traits::static_query<T, unsequenced_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        traits::static_query<T, sequenced_t>::is_valid
       >::type* = 0) ASIO_NOEXCEPT
   {
     return traits::static_query<T, sequenced_t>::value();
@@ -286,10 +296,18 @@ struct bulk_guarantee_t
   static_query(
       typename enable_if<
         !traits::query_static_constexpr_member<T, bulk_guarantee_t>::is_valid
-          && !traits::query_member<T, bulk_guarantee_t>::is_valid
-          && !traits::static_query<T, unsequenced_t>::is_valid
-          && !traits::static_query<T, sequenced_t>::is_valid
-          && traits::static_query<T, parallel_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        !traits::query_member<T, bulk_guarantee_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        !traits::static_query<T, unsequenced_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        !traits::static_query<T, sequenced_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        traits::static_query<T, parallel_t>::is_valid
       >::type* = 0) ASIO_NOEXCEPT
   {
     return traits::static_query<T, parallel_t>::value();
@@ -344,7 +362,9 @@ struct bulk_guarantee_t
       const Executor& ex, convertible_from_bulk_guarantee_t,
       typename enable_if<
         !can_query<const Executor&, unsequenced_t>::value
-          && can_query<const Executor&, sequenced_t>::value
+      >::type* = 0,
+      typename enable_if<
+        can_query<const Executor&, sequenced_t>::value
       >::type* = 0)
 #if !defined(__clang__) // Clang crashes if noexcept is used here.
 #if defined(ASIO_MSVC) // Visual C++ wants the type to be qualified.
@@ -365,8 +385,12 @@ struct bulk_guarantee_t
       const Executor& ex, convertible_from_bulk_guarantee_t,
       typename enable_if<
         !can_query<const Executor&, unsequenced_t>::value
-          && !can_query<const Executor&, sequenced_t>::value
-          && can_query<const Executor&, parallel_t>::value
+      >::type* = 0,
+      typename enable_if<
+        !can_query<const Executor&, sequenced_t>::value
+      >::type* = 0,
+      typename enable_if<
+        can_query<const Executor&, parallel_t>::value
       >::type* = 0)
 #if !defined(__clang__) // Clang crashes if noexcept is used here.
 #if defined(ASIO_MSVC) // Visual C++ wants the type to be qualified.
@@ -463,10 +487,18 @@ struct unsequenced_t
   static ASIO_CONSTEXPR unsequenced_t static_query(
       typename enable_if<
         !traits::query_static_constexpr_member<T, unsequenced_t>::is_valid
-          && !traits::query_member<T, unsequenced_t>::is_valid
-          && !traits::query_free<T, unsequenced_t>::is_valid
-          && !can_query<T, sequenced_t<I> >::value
-          && !can_query<T, parallel_t<I> >::value
+      >::type* = 0,
+      typename enable_if<
+        !traits::query_member<T, unsequenced_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        !traits::query_free<T, unsequenced_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        !can_query<T, sequenced_t<I> >::value
+      >::type* = 0,
+      typename enable_if<
+        !can_query<T, parallel_t<I> >::value
       >::type* = 0) ASIO_NOEXCEPT
   {
     return unsequenced_t();
diff --git a/asio/include/asio/execution/connect.hpp b/asio/include/asio/execution/connect.hpp
index 47dd58b6..17a924bc 100644
--- a/asio/include/asio/execution/connect.hpp
+++ b/asio/include/asio/execution/connect.hpp
@@ -157,7 +157,8 @@ enum overload_type
   ill_formed
 };
 
-template <typename S, typename R, typename = void>
+template <typename S, typename R, typename = void,
+   typename = void, typename = void, typename = void>
 struct call_traits
 {
   ASIO_STATIC_CONSTEXPR(overload_type, overload = ill_formed);
@@ -168,13 +169,13 @@ struct call_traits
 template <typename S, typename R>
 struct call_traits<S, void(R),
   typename enable_if<
-    (
-      connect_member<S, R>::is_valid
-      &&
-      is_operation_state<typename connect_member<S, R>::result_type>::value
-      &&
-      is_sender<typename remove_cvref<S>::type>::value
-    )
+    connect_member<S, R>::is_valid
+  >::type,
+  typename enable_if<
+    is_operation_state<typename connect_member<S, R>::result_type>::value
+  >::type,
+  typename enable_if<
+    is_sender<typename remove_cvref<S>::type>::value
   >::type> :
   connect_member<S, R>
 {
@@ -184,15 +185,16 @@ struct call_traits<S, void(R),
 template <typename S, typename R>
 struct call_traits<S, void(R),
   typename enable_if<
-    (
-      !connect_member<S, R>::is_valid
-      &&
-      connect_free<S, R>::is_valid
-      &&
-      is_operation_state<typename connect_free<S, R>::result_type>::value
-      &&
-      is_sender<typename remove_cvref<S>::type>::value
-    )
+    !connect_member<S, R>::is_valid
+  >::type,
+  typename enable_if<
+    connect_free<S, R>::is_valid
+  >::type,
+  typename enable_if<
+    is_operation_state<typename connect_free<S, R>::result_type>::value
+  >::type,
+  typename enable_if<
+    is_sender<typename remove_cvref<S>::type>::value
   >::type> :
   connect_free<S, R>
 {
@@ -202,24 +204,25 @@ struct call_traits<S, void(R),
 template <typename S, typename R>
 struct call_traits<S, void(R),
   typename enable_if<
-    (
-      !connect_member<S, R>::is_valid
-      &&
-      !connect_free<S, R>::is_valid
-      &&
-      is_receiver<R>::value
-      &&
-      conditional<
-        !is_as_receiver<
-          typename remove_cvref<R>::type
-        >::value,
-        is_executor_of<
-          typename remove_cvref<S>::type,
-          as_invocable<typename remove_cvref<R>::type, S>
-        >,
-        false_type
-      >::type::value
-    )
+    !connect_member<S, R>::is_valid
+  >::type,
+  typename enable_if<
+    !connect_free<S, R>::is_valid
+  >::type,
+  typename enable_if<
+    is_receiver<R>::value
+  >::type,
+  typename enable_if<
+    conditional<
+      !is_as_receiver<
+        typename remove_cvref<R>::type
+      >::value,
+      is_executor_of<
+        typename remove_cvref<S>::type,
+        as_invocable<typename remove_cvref<R>::type, S>
+      >,
+      false_type
+    >::type::value
   >::type>
 {
   ASIO_STATIC_CONSTEXPR(overload_type, overload = adapter);
diff --git a/asio/include/asio/execution/context_as.hpp b/asio/include/asio/execution/context_as.hpp
index b4a93a51..f39376cf 100644
--- a/asio/include/asio/execution/context_as.hpp
+++ b/asio/include/asio/execution/context_as.hpp
@@ -120,7 +120,9 @@ struct context_as_t
       const Executor& ex, const context_as_t<U>&,
       typename enable_if<
         is_same<T, U>::value
-          && can_query<const Executor&, const context_t&>::value
+      >::type* = 0,
+      typename enable_if<
+        can_query<const Executor&, const context_t&>::value
       >::type* = 0)
 #if !defined(__clang__) // Clang crashes if noexcept is used here.
 #if defined(ASIO_MSVC) // Visual C++ wants the type to be qualified.
diff --git a/asio/include/asio/execution/execute.hpp b/asio/include/asio/execution/execute.hpp
index 277cd18a..483efbbc 100644
--- a/asio/include/asio/execution/execute.hpp
+++ b/asio/include/asio/execution/execute.hpp
@@ -86,7 +86,6 @@ void submit_helper(ASIO_MOVE_ARG(S) s, ASIO_MOVE_ARG(R) r);
 } // namespace asio
 namespace asio_execution_execute_fn {
 
-using asio::conditional;
 using asio::decay;
 using asio::declval;
 using asio::enable_if;
@@ -98,6 +97,7 @@ using asio::result_of;
 using asio::traits::execute_free;
 using asio::traits::execute_member;
 using asio::true_type;
+using asio::void_type;
 
 void execute();
 
@@ -109,7 +109,8 @@ enum overload_type
   ill_formed
 };
 
-template <typename T, typename F, typename = void>
+template <typename T, typename F, typename = void, typename = void,
+    typename = void, typename = void, typename = void>
 struct call_traits
 {
   ASIO_STATIC_CONSTEXPR(overload_type, overload = ill_formed);
@@ -118,9 +119,7 @@ struct call_traits
 template <typename T, typename F>
 struct call_traits<T, void(F),
   typename enable_if<
-    (
       execute_member<T, F>::is_valid
-    )
   >::type> :
   execute_member<T, F>
 {
@@ -130,11 +129,10 @@ struct call_traits<T, void(F),
 template <typename T, typename F>
 struct call_traits<T, void(F),
   typename enable_if<
-    (
-      !execute_member<T, F>::is_valid
-      &&
-      execute_free<T, F>::is_valid
-    )
+    !execute_member<T, F>::is_valid
+  >::type,
+  typename enable_if<
+    execute_free<T, F>::is_valid
   >::type> :
   execute_free<T, F>
 {
@@ -144,26 +142,19 @@ struct call_traits<T, void(F),
 template <typename T, typename F>
 struct call_traits<T, void(F),
   typename enable_if<
-    (
-      !execute_member<T, F>::is_valid
-      &&
-      !execute_free<T, F>::is_valid
-      &&
-      conditional<true, true_type,
-       typename result_of<typename decay<F>::type&()>::type
-      >::type::value
-      &&
-      conditional<
-        !is_as_invocable<
-          typename decay<F>::type
-        >::value,
-        is_sender_to<
-          T,
-          as_receiver<typename decay<F>::type, T>
-        >,
-        false_type
-      >::type::value
-    )
+    !execute_member<T, F>::is_valid
+  >::type,
+  typename enable_if<
+    !execute_free<T, F>::is_valid
+  >::type,
+  typename void_type<
+   typename result_of<typename decay<F>::type&()>::type
+  >::type,
+  typename enable_if<
+    !is_as_invocable<typename decay<F>::type>::value
+  >::type,
+  typename enable_if<
+    is_sender_to<T, as_receiver<typename decay<F>::type, T> >::value
   >::type>
 {
   ASIO_STATIC_CONSTEXPR(overload_type, overload = adapter);
diff --git a/asio/include/asio/execution/executor.hpp b/asio/include/asio/execution/executor.hpp
index fb64ebae..6a9006b0 100644
--- a/asio/include/asio/execution/executor.hpp
+++ b/asio/include/asio/execution/executor.hpp
@@ -35,34 +35,48 @@ namespace asio {
 namespace execution {
 namespace detail {
 
-template <typename T, typename F>
-struct is_executor_of_impl_base :
-  integral_constant<bool,
-    conditional<true, true_type,
-        typename result_of<typename decay<F>::type&()>::type
-      >::type::value
-      && is_constructible<typename decay<F>::type, F>::value
-      && is_move_constructible<typename decay<F>::type>::value
-#if defined(ASIO_HAS_NOEXCEPT)
-      && is_nothrow_copy_constructible<T>::value
-      && is_nothrow_destructible<T>::value
-#else // defined(ASIO_HAS_NOEXCEPT)
-      && is_copy_constructible<T>::value
-      && is_destructible<T>::value
-#endif // defined(ASIO_HAS_NOEXCEPT)
-      && traits::equality_comparable<T>::is_valid
-      && traits::equality_comparable<T>::is_noexcept
-  >
+template <typename T, typename F,
+    typename = void, typename = void, typename = void, typename = void,
+    typename = void, typename = void, typename = void, typename = void>
+struct is_executor_of_impl : false_type
 {
 };
 
 template <typename T, typename F>
-struct is_executor_of_impl :
-  conditional<
-    can_execute<typename add_const<T>::type, F>::value,
-    is_executor_of_impl_base<T, F>,
-    false_type
-  >::type
+struct is_executor_of_impl<T, F,
+  typename enable_if<
+    can_execute<typename add_const<T>::type, F>::value
+  >::type,
+  typename void_type<
+    typename result_of<typename decay<F>::type&()>::type
+  >::type,
+  typename enable_if<
+    is_constructible<typename decay<F>::type, F>::value
+  >::type,
+  typename enable_if<
+    is_move_constructible<typename decay<F>::type>::value
+  >::type,
+#if defined(ASIO_HAS_NOEXCEPT)
+  typename enable_if<
+    is_nothrow_copy_constructible<T>::value
+  >::type,
+  typename enable_if<
+    is_nothrow_destructible<T>::value
+  >::type,
+#else // defined(ASIO_HAS_NOEXCEPT)
+  typename enable_if<
+    is_copy_constructible<T>::value
+  >::type,
+  typename enable_if<
+    is_destructible<T>::value
+  >::type,
+#endif // defined(ASIO_HAS_NOEXCEPT)
+  typename enable_if<
+    traits::equality_comparable<T>::is_valid
+  >::type,
+  typename enable_if<
+    traits::equality_comparable<T>::is_noexcept
+  >::type> : true_type
 {
 };
 
diff --git a/asio/include/asio/execution/mapping.hpp b/asio/include/asio/execution/mapping.hpp
index 43a27330..205d8141 100644
--- a/asio/include/asio/execution/mapping.hpp
+++ b/asio/include/asio/execution/mapping.hpp
@@ -254,8 +254,12 @@ struct mapping_t
   static_query(
       typename enable_if<
         !traits::query_static_constexpr_member<T, mapping_t>::is_valid
-          && !traits::query_member<T, mapping_t>::is_valid
-          && traits::static_query<T, thread_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        !traits::query_member<T, mapping_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        traits::static_query<T, thread_t>::is_valid
       >::type* = 0) ASIO_NOEXCEPT
   {
     return traits::static_query<T, thread_t>::value();
@@ -267,9 +271,15 @@ struct mapping_t
   static_query(
       typename enable_if<
         !traits::query_static_constexpr_member<T, mapping_t>::is_valid
-          && !traits::query_member<T, mapping_t>::is_valid
-          && !traits::static_query<T, thread_t>::is_valid
-          && traits::static_query<T, new_thread_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        !traits::query_member<T, mapping_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        !traits::static_query<T, thread_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        traits::static_query<T, new_thread_t>::is_valid
       >::type* = 0) ASIO_NOEXCEPT
   {
     return traits::static_query<T, new_thread_t>::value();
@@ -281,10 +291,18 @@ struct mapping_t
   static_query(
       typename enable_if<
         !traits::query_static_constexpr_member<T, mapping_t>::is_valid
-          && !traits::query_member<T, mapping_t>::is_valid
-          && !traits::static_query<T, thread_t>::is_valid
-          && !traits::static_query<T, new_thread_t>::is_valid
-          && traits::static_query<T, other_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        !traits::query_member<T, mapping_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        !traits::static_query<T, thread_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        !traits::static_query<T, new_thread_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        traits::static_query<T, other_t>::is_valid
       >::type* = 0) ASIO_NOEXCEPT
   {
     return traits::static_query<T, other_t>::value();
@@ -337,7 +355,9 @@ struct mapping_t
       const Executor& ex, convertible_from_mapping_t,
       typename enable_if<
         !can_query<const Executor&, thread_t>::value
-          && can_query<const Executor&, new_thread_t>::value
+      >::type* = 0,
+      typename enable_if<
+        can_query<const Executor&, new_thread_t>::value
       >::type* = 0)
 #if !defined(__clang__) // Clang crashes if noexcept is used here.
 #if defined(ASIO_MSVC) // Visual C++ wants the type to be qualified.
@@ -357,8 +377,12 @@ struct mapping_t
       const Executor& ex, convertible_from_mapping_t,
       typename enable_if<
         !can_query<const Executor&, thread_t>::value
-          && !can_query<const Executor&, new_thread_t>::value
-          && can_query<const Executor&, other_t>::value
+      >::type* = 0,
+      typename enable_if<
+        !can_query<const Executor&, new_thread_t>::value
+      >::type* = 0,
+      typename enable_if<
+        can_query<const Executor&, other_t>::value
       >::type* = 0)
 #if !defined(__clang__) // Clang crashes if noexcept is used here.
 #if defined(ASIO_MSVC) // Visual C++ wants the type to be qualified.
@@ -452,10 +476,18 @@ struct thread_t
   static ASIO_CONSTEXPR thread_t static_query(
       typename enable_if<
         !traits::query_static_constexpr_member<T, thread_t>::is_valid
-          && !traits::query_member<T, thread_t>::is_valid
-          && !traits::query_free<T, thread_t>::is_valid
-          && !can_query<T, new_thread_t<I> >::value
-          && !can_query<T, other_t<I> >::value
+      >::type* = 0,
+      typename enable_if<
+        !traits::query_member<T, thread_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        !traits::query_free<T, thread_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        !can_query<T, new_thread_t<I> >::value
+      >::type* = 0,
+      typename enable_if<
+        !can_query<T, other_t<I> >::value
       >::type* = 0) ASIO_NOEXCEPT
   {
     return thread_t();
diff --git a/asio/include/asio/execution/outstanding_work.hpp b/asio/include/asio/execution/outstanding_work.hpp
index da585476..edb9c0b2 100644
--- a/asio/include/asio/execution/outstanding_work.hpp
+++ b/asio/include/asio/execution/outstanding_work.hpp
@@ -219,8 +219,12 @@ struct outstanding_work_t
       typename enable_if<
         !traits::query_static_constexpr_member<
             T, outstanding_work_t>::is_valid
-          && !traits::query_member<T, outstanding_work_t>::is_valid
-          && traits::static_query<T, untracked_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        !traits::query_member<T, outstanding_work_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        traits::static_query<T, untracked_t>::is_valid
       >::type* = 0) ASIO_NOEXCEPT
   {
     return traits::static_query<T, untracked_t>::value();
@@ -233,9 +237,15 @@ struct outstanding_work_t
       typename enable_if<
         !traits::query_static_constexpr_member<
             T, outstanding_work_t>::is_valid
-          && !traits::query_member<T, outstanding_work_t>::is_valid
-          && !traits::static_query<T, untracked_t>::is_valid
-          && traits::static_query<T, tracked_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        !traits::query_member<T, outstanding_work_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        !traits::static_query<T, untracked_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        traits::static_query<T, tracked_t>::is_valid
       >::type* = 0) ASIO_NOEXCEPT
   {
     return traits::static_query<T, tracked_t>::value();
@@ -292,7 +302,9 @@ struct outstanding_work_t
       const Executor& ex, convertible_from_outstanding_work_t,
       typename enable_if<
         !can_query<const Executor&, untracked_t>::value
-          && can_query<const Executor&, tracked_t>::value
+      >::type* = 0,
+      typename enable_if<
+        can_query<const Executor&, tracked_t>::value
       >::type* = 0)
 #if !defined(__clang__) // Clang crashes if noexcept is used here.
 #if defined(ASIO_MSVC) // Visual C++ wants the type to be qualified.
@@ -385,9 +397,15 @@ struct untracked_t
   static ASIO_CONSTEXPR untracked_t static_query(
       typename enable_if<
         !traits::query_static_constexpr_member<T, untracked_t>::is_valid
-          && !traits::query_member<T, untracked_t>::is_valid
-          && !traits::query_free<T, untracked_t>::is_valid
-          && !can_query<T, tracked_t<I> >::value
+      >::type* = 0,
+      typename enable_if<
+        !traits::query_member<T, untracked_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        !traits::query_free<T, untracked_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        !can_query<T, tracked_t<I> >::value
       >::type* = 0) ASIO_NOEXCEPT
   {
     return untracked_t();
diff --git a/asio/include/asio/execution/prefer_only.hpp b/asio/include/asio/execution/prefer_only.hpp
index 9823dade..5e0bba8f 100644
--- a/asio/include/asio/execution/prefer_only.hpp
+++ b/asio/include/asio/execution/prefer_only.hpp
@@ -220,7 +220,9 @@ struct prefer_only :
   prefer(const Executor& ex, const prefer_only<Property>& p,
       typename enable_if<
         is_same<Property, InnerProperty>::value
-          && can_prefer<const Executor&, const InnerProperty&>::value
+      >::type* = 0,
+      typename enable_if<
+        can_prefer<const Executor&, const InnerProperty&>::value
       >::type* = 0)
 #if !defined(ASIO_MSVC) \
   && !defined(__clang__) // Clang crashes if noexcept is used here.
@@ -238,7 +240,9 @@ struct prefer_only :
   query(const Executor& ex, const prefer_only<Property>& p,
       typename enable_if<
         is_same<Property, InnerProperty>::value
-          && can_query<const Executor&, const InnerProperty&>::value
+      >::type* = 0,
+      typename enable_if<
+        can_query<const Executor&, const InnerProperty&>::value
       >::type* = 0)
 #if !defined(ASIO_MSVC) \
   && !defined(__clang__) // Clang crashes if noexcept is used here.
diff --git a/asio/include/asio/execution/relationship.hpp b/asio/include/asio/execution/relationship.hpp
index 35f07494..fff8d38a 100644
--- a/asio/include/asio/execution/relationship.hpp
+++ b/asio/include/asio/execution/relationship.hpp
@@ -218,8 +218,12 @@ struct relationship_t
       typename enable_if<
         !traits::query_static_constexpr_member<
             T, relationship_t>::is_valid
-          && !traits::query_member<T, relationship_t>::is_valid
-          && traits::static_query<T, fork_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        !traits::query_member<T, relationship_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        traits::static_query<T, fork_t>::is_valid
       >::type* = 0) ASIO_NOEXCEPT
   {
     return traits::static_query<T, fork_t>::value();
@@ -232,9 +236,15 @@ struct relationship_t
       typename enable_if<
         !traits::query_static_constexpr_member<
             T, relationship_t>::is_valid
-          && !traits::query_member<T, relationship_t>::is_valid
-          && !traits::static_query<T, fork_t>::is_valid
-          && traits::static_query<T, continuation_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        !traits::query_member<T, relationship_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        !traits::static_query<T, fork_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        traits::static_query<T, continuation_t>::is_valid
       >::type* = 0) ASIO_NOEXCEPT
   {
     return traits::static_query<T, continuation_t>::value();
@@ -290,7 +300,9 @@ struct relationship_t
       const Executor& ex, convertible_from_relationship_t,
       typename enable_if<
         !can_query<const Executor&, fork_t>::value
-          && can_query<const Executor&, continuation_t>::value
+      >::type* = 0,
+      typename enable_if<
+        can_query<const Executor&, continuation_t>::value
       >::type* = 0)
 #if !defined(__clang__) // Clang crashes if noexcept is used here.
 #if defined(ASIO_MSVC) // Visual C++ wants the type to be qualified.
@@ -383,9 +395,15 @@ struct fork_t
   static ASIO_CONSTEXPR fork_t static_query(
       typename enable_if<
         !traits::query_static_constexpr_member<T, fork_t>::is_valid
-          && !traits::query_member<T, fork_t>::is_valid
-          && !traits::query_free<T, fork_t>::is_valid
-          && !can_query<T, continuation_t<I> >::value
+      >::type* = 0,
+      typename enable_if<
+        !traits::query_member<T, fork_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        !traits::query_free<T, fork_t>::is_valid
+      >::type* = 0,
+      typename enable_if<
+        !can_query<T, continuation_t<I> >::value
       >::type* = 0) ASIO_NOEXCEPT
   {
     return fork_t();
diff --git a/asio/include/asio/execution/schedule.hpp b/asio/include/asio/execution/schedule.hpp
index 1262735f..95ecc9f1 100644
--- a/asio/include/asio/execution/schedule.hpp
+++ b/asio/include/asio/execution/schedule.hpp
@@ -86,7 +86,7 @@ enum overload_type
   ill_formed
 };
 
-template <typename S, typename = void>
+template <typename S, typename = void, typename = void, typename = void>
 struct call_traits
 {
   ASIO_STATIC_CONSTEXPR(overload_type, overload = ill_formed);
@@ -97,9 +97,7 @@ struct call_traits
 template <typename S>
 struct call_traits<S,
   typename enable_if<
-    (
-      schedule_member<S>::is_valid
-    )
+    schedule_member<S>::is_valid
   >::type> :
   schedule_member<S>
 {
@@ -109,11 +107,10 @@ struct call_traits<S,
 template <typename S>
 struct call_traits<S,
   typename enable_if<
-    (
-      !schedule_member<S>::is_valid
-      &&
-      schedule_free<S>::is_valid
-    )
+    !schedule_member<S>::is_valid
+  >::type,
+  typename enable_if<
+    schedule_free<S>::is_valid
   >::type> :
   schedule_free<S>
 {
@@ -123,13 +120,13 @@ struct call_traits<S,
 template <typename S>
 struct call_traits<S,
   typename enable_if<
-    (
-      !schedule_member<S>::is_valid
-      &&
-      !schedule_free<S>::is_valid
-      &&
-      is_executor<typename decay<S>::type>::value
-    )
+    !schedule_member<S>::is_valid
+  >::type,
+  typename enable_if<
+    !schedule_free<S>::is_valid
+  >::type,
+  typename enable_if<
+    is_executor<typename decay<S>::type>::value
   >::type>
 {
   ASIO_STATIC_CONSTEXPR(overload_type, overload = identity);
diff --git a/asio/include/asio/execution/set_done.hpp b/asio/include/asio/execution/set_done.hpp
index 5b45b7eb..1ec0d556 100644
--- a/asio/include/asio/execution/set_done.hpp
+++ b/asio/include/asio/execution/set_done.hpp
@@ -83,7 +83,7 @@ enum overload_type
   ill_formed
 };
 
-template <typename R, typename = void>
+template <typename R, typename = void, typename = void>
 struct call_traits
 {
   ASIO_STATIC_CONSTEXPR(overload_type, overload = ill_formed);
@@ -94,9 +94,7 @@ struct call_traits
 template <typename R>
 struct call_traits<R,
   typename enable_if<
-    (
-      set_done_member<R>::is_valid
-    )
+    set_done_member<R>::is_valid
   >::type> :
   set_done_member<R>
 {
@@ -106,11 +104,10 @@ struct call_traits<R,
 template <typename R>
 struct call_traits<R,
   typename enable_if<
-    (
-      !set_done_member<R>::is_valid
-      &&
-      set_done_free<R>::is_valid
-    )
+    !set_done_member<R>::is_valid
+  >::type,
+  typename enable_if<
+    set_done_free<R>::is_valid
   >::type> :
   set_done_free<R>
 {
diff --git a/asio/include/asio/execution/set_error.hpp b/asio/include/asio/execution/set_error.hpp
index aa701465..f09fded6 100644
--- a/asio/include/asio/execution/set_error.hpp
+++ b/asio/include/asio/execution/set_error.hpp
@@ -83,7 +83,7 @@ enum overload_type
   ill_formed
 };
 
-template <typename R, typename E, typename = void>
+template <typename R, typename E, typename = void, typename = void>
 struct call_traits
 {
   ASIO_STATIC_CONSTEXPR(overload_type, overload = ill_formed);
@@ -94,9 +94,7 @@ struct call_traits
 template <typename R, typename E>
 struct call_traits<R, void(E),
   typename enable_if<
-    (
-      set_error_member<R, E>::is_valid
-    )
+    set_error_member<R, E>::is_valid
   >::type> :
   set_error_member<R, E>
 {
@@ -106,11 +104,10 @@ struct call_traits<R, void(E),
 template <typename R, typename E>
 struct call_traits<R, void(E),
   typename enable_if<
-    (
-      !set_error_member<R, E>::is_valid
-      &&
-      set_error_free<R, E>::is_valid
-    )
+    !set_error_member<R, E>::is_valid
+  >::type,
+  typename enable_if<
+    set_error_free<R, E>::is_valid
   >::type> :
   set_error_free<R, E>
 {
diff --git a/asio/include/asio/execution/set_value.hpp b/asio/include/asio/execution/set_value.hpp
index f42b19db..a6834b88 100644
--- a/asio/include/asio/execution/set_value.hpp
+++ b/asio/include/asio/execution/set_value.hpp
@@ -86,7 +86,7 @@ enum overload_type
   ill_formed
 };
 
-template <typename R, typename Vs, typename = void>
+template <typename R, typename Vs, typename = void, typename = void>
 struct call_traits
 {
   ASIO_STATIC_CONSTEXPR(overload_type, overload = ill_formed);
@@ -97,9 +97,7 @@ struct call_traits
 template <typename R, typename Vs>
 struct call_traits<R, Vs,
   typename enable_if<
-    (
-      set_value_member<R, Vs>::is_valid
-    )
+    set_value_member<R, Vs>::is_valid
   >::type> :
   set_value_member<R, Vs>
 {
@@ -109,11 +107,10 @@ struct call_traits<R, Vs,
 template <typename R, typename Vs>
 struct call_traits<R, Vs,
   typename enable_if<
-    (
-      !set_value_member<R, Vs>::is_valid
-      &&
-      set_value_free<R, Vs>::is_valid
-    )
+    !set_value_member<R, Vs>::is_valid
+  >::type,
+  typename enable_if<
+    set_value_free<R, Vs>::is_valid
   >::type> :
   set_value_free<R, Vs>
 {
diff --git a/asio/include/asio/execution/start.hpp b/asio/include/asio/execution/start.hpp
index 194be147..49e3d1c3 100644
--- a/asio/include/asio/execution/start.hpp
+++ b/asio/include/asio/execution/start.hpp
@@ -80,7 +80,7 @@ enum overload_type
   ill_formed
 };
 
-template <typename R, typename = void>
+template <typename R, typename = void, typename = void>
 struct call_traits
 {
   ASIO_STATIC_CONSTEXPR(overload_type, overload = ill_formed);
@@ -91,9 +91,7 @@ struct call_traits
 template <typename R>
 struct call_traits<R,
   typename enable_if<
-    (
-      start_member<R>::is_valid
-    )
+    start_member<R>::is_valid
   >::type> :
   start_member<R>
 {
@@ -103,11 +101,10 @@ struct call_traits<R,
 template <typename R>
 struct call_traits<R,
   typename enable_if<
-    (
-      !start_member<R>::is_valid
-      &&
-      start_free<R>::is_valid
-    )
+    !start_member<R>::is_valid
+  >::type,
+  typename enable_if<
+    start_free<R>::is_valid
   >::type> :
   start_free<R>
 {
diff --git a/asio/include/asio/execution/submit.hpp b/asio/include/asio/execution/submit.hpp
index 719d27eb..39c05e97 100644
--- a/asio/include/asio/execution/submit.hpp
+++ b/asio/include/asio/execution/submit.hpp
@@ -125,7 +125,8 @@ enum overload_type
   ill_formed
 };
 
-template <typename S, typename R, typename = void>
+template <typename S, typename R, typename = void,
+    typename = void, typename = void>
 struct call_traits
 {
   ASIO_STATIC_CONSTEXPR(overload_type, overload = ill_formed);
@@ -136,11 +137,10 @@ struct call_traits
 template <typename S, typename R>
 struct call_traits<S, void(R),
   typename enable_if<
-    (
-      submit_member<S, R>::is_valid
-      &&
-      is_sender_to<S, R>::value
-    )
+    submit_member<S, R>::is_valid
+  >::type,
+  typename enable_if<
+    is_sender_to<S, R>::value
   >::type> :
   submit_member<S, R>
 {
@@ -150,13 +150,13 @@ struct call_traits<S, void(R),
 template <typename S, typename R>
 struct call_traits<S, void(R),
   typename enable_if<
-    (
-      !submit_member<S, R>::is_valid
-      &&
-      submit_free<S, R>::is_valid
-      &&
-      is_sender_to<S, R>::value
-    )
+    !submit_member<S, R>::is_valid
+  >::type,
+  typename enable_if<
+    submit_free<S, R>::is_valid
+  >::type,
+  typename enable_if<
+    is_sender_to<S, R>::value
   >::type> :
   submit_free<S, R>
 {
@@ -166,13 +166,13 @@ struct call_traits<S, void(R),
 template <typename S, typename R>
 struct call_traits<S, void(R),
   typename enable_if<
-    (
-      !submit_member<S, R>::is_valid
-      &&
-      !submit_free<S, R>::is_valid
-      &&
-      is_sender_to<S, R>::value
-    )
+    !submit_member<S, R>::is_valid
+  >::type,
+  typename enable_if<
+    !submit_free<S, R>::is_valid
+  >::type,
+  typename enable_if<
+    is_sender_to<S, R>::value
   >::type>
 {
   ASIO_STATIC_CONSTEXPR(overload_type, overload = adapter);
diff --git a/asio/include/asio/executor_work_guard.hpp b/asio/include/asio/executor_work_guard.hpp
index 5612f569..510939e7 100644
--- a/asio/include/asio/executor_work_guard.hpp
+++ b/asio/include/asio/executor_work_guard.hpp
@@ -31,7 +31,7 @@ namespace asio {
 #if !defined(ASIO_EXECUTOR_WORK_GUARD_DECL)
 #define ASIO_EXECUTOR_WORK_GUARD_DECL
 
-template <typename Executor, typename = void>
+template <typename Executor, typename = void, typename = void>
 class executor_work_guard;
 
 #endif // !defined(ASIO_EXECUTOR_WORK_GUARD_DECL)
@@ -41,7 +41,7 @@ class executor_work_guard;
 #if defined(GENERATING_DOCUMENTATION)
 template <typename Executor>
 #else // defined(GENERATING_DOCUMENTATION)
-template <typename Executor, typename>
+template <typename Executor, typename, typename>
 #endif // defined(GENERATING_DOCUMENTATION)
 class executor_work_guard
 {
@@ -129,7 +129,10 @@ private:
 template <typename Executor>
 class executor_work_guard<Executor,
     typename enable_if<
-      !is_executor<Executor>::value && execution::is_executor<Executor>::value
+      !is_executor<Executor>::value
+    >::type,
+    typename enable_if<
+      execution::is_executor<Executor>::value
     >::type>
 {
 public:
@@ -241,9 +244,14 @@ template <typename T>
 inline executor_work_guard<typename associated_executor<T>::type>
 make_work_guard(const T& t,
     typename enable_if<
-      !is_executor<T>::value && !execution::is_executor<T>::value
-        && !is_convertible<T&, execution_context&
-    >::value>::type* = 0)
+      !is_executor<T>::value
+    >::type* = 0,
+    typename enable_if<
+      !execution::is_executor<T>::value
+    >::type* = 0,
+    typename enable_if<
+      !is_convertible<T&, execution_context&>::value
+    >::type* = 0)
 {
   return executor_work_guard<typename associated_executor<T>::type>(
       associated_executor<T>::get(t));
@@ -267,9 +275,16 @@ inline executor_work_guard<typename associated_executor<T,
   typename ExecutionContext::executor_type>::type>
 make_work_guard(const T& t, ExecutionContext& ctx,
     typename enable_if<
-      !is_executor<T>::value && !execution::is_executor<T>::value
-        && !is_convertible<T&, execution_context&>::value
-        && is_convertible<ExecutionContext&, execution_context&>::value
+      !is_executor<T>::value
+    >::type* = 0,
+    typename enable_if<
+      !execution::is_executor<T>::value
+    >::type* = 0,
+    typename enable_if<
+      !is_convertible<T&, execution_context&>::value
+    >::type* = 0,
+    typename enable_if<
+      is_convertible<ExecutionContext&, execution_context&>::value
     >::type* = 0)
 {
   return executor_work_guard<typename associated_executor<T,
diff --git a/asio/include/asio/impl/compose.hpp b/asio/include/asio/impl/compose.hpp
index 1a2a6e06..1c453dc9 100644
--- a/asio/include/asio/impl/compose.hpp
+++ b/asio/include/asio/impl/compose.hpp
@@ -500,7 +500,9 @@ namespace detail
   get_composed_io_executor(IoObject& io_object,
       typename enable_if<
         !is_executor<IoObject>::value
-          && !execution::is_executor<IoObject>::value
+      >::type* = 0,
+      typename enable_if<
+        !execution::is_executor<IoObject>::value
       >::type* = 0)
   {
     return io_object.get_executor();
diff --git a/asio/include/asio/impl/defer.hpp b/asio/include/asio/impl/defer.hpp
index 28e0a652..0288eb4c 100644
--- a/asio/include/asio/impl/defer.hpp
+++ b/asio/include/asio/impl/defer.hpp
@@ -103,7 +103,8 @@ public:
         execution::is_executor<
           typename conditional<true, executor_type, CompletionHandler>::type
         >::value
-        &&
+      >::type* = 0,
+      typename enable_if<
         !detail::is_work_dispatcher_required<
           typename decay<CompletionHandler>::type,
           Executor
@@ -129,7 +130,8 @@ public:
         execution::is_executor<
           typename conditional<true, executor_type, CompletionHandler>::type
         >::value
-        &&
+      >::type* = 0,
+      typename enable_if<
         detail::is_work_dispatcher_required<
           typename decay<CompletionHandler>::type,
           Executor
@@ -160,7 +162,8 @@ public:
         !execution::is_executor<
           typename conditional<true, executor_type, CompletionHandler>::type
         >::value
-        &&
+      >::type* = 0,
+      typename enable_if<
         !detail::is_work_dispatcher_required<
           typename decay<CompletionHandler>::type,
           Executor
@@ -181,7 +184,8 @@ public:
         !execution::is_executor<
           typename conditional<true, executor_type, CompletionHandler>::type
         >::value
-        &&
+      >::type* = 0,
+      typename enable_if<
         detail::is_work_dispatcher_required<
           typename decay<CompletionHandler>::type,
           Executor
diff --git a/asio/include/asio/impl/dispatch.hpp b/asio/include/asio/impl/dispatch.hpp
index 9a542d1e..a4f4ca95 100644
--- a/asio/include/asio/impl/dispatch.hpp
+++ b/asio/include/asio/impl/dispatch.hpp
@@ -100,7 +100,8 @@ public:
         execution::is_executor<
           typename conditional<true, executor_type, CompletionHandler>::type
         >::value
-        &&
+      >::type* = 0,
+      typename enable_if<
         !detail::is_work_dispatcher_required<
           typename decay<CompletionHandler>::type,
           Executor
@@ -125,7 +126,8 @@ public:
         execution::is_executor<
           typename conditional<true, executor_type, CompletionHandler>::type
         >::value
-        &&
+      >::type* = 0,
+      typename enable_if<
         detail::is_work_dispatcher_required<
           typename decay<CompletionHandler>::type,
           Executor
@@ -155,7 +157,8 @@ public:
         !execution::is_executor<
           typename conditional<true, executor_type, CompletionHandler>::type
         >::value
-        &&
+      >::type* = 0,
+      typename enable_if<
         !detail::is_work_dispatcher_required<
           typename decay<CompletionHandler>::type,
           Executor
@@ -176,7 +179,8 @@ public:
         !execution::is_executor<
           typename conditional<true, executor_type, CompletionHandler>::type
         >::value
-        &&
+      >::type* = 0,
+      typename enable_if<
         detail::is_work_dispatcher_required<
           typename decay<CompletionHandler>::type,
           Executor
diff --git a/asio/include/asio/impl/post.hpp b/asio/include/asio/impl/post.hpp
index d1d39116..b5511ace 100644
--- a/asio/include/asio/impl/post.hpp
+++ b/asio/include/asio/impl/post.hpp
@@ -103,7 +103,8 @@ public:
         execution::is_executor<
           typename conditional<true, executor_type, CompletionHandler>::type
         >::value
-        &&
+      >::type* = 0,
+      typename enable_if<
         !detail::is_work_dispatcher_required<
           typename decay<CompletionHandler>::type,
           Executor
@@ -129,7 +130,8 @@ public:
         execution::is_executor<
           typename conditional<true, executor_type, CompletionHandler>::type
         >::value
-        &&
+      >::type* = 0,
+      typename enable_if<
         detail::is_work_dispatcher_required<
           typename decay<CompletionHandler>::type,
           Executor
@@ -160,7 +162,8 @@ public:
         !execution::is_executor<
           typename conditional<true, executor_type, CompletionHandler>::type
         >::value
-        &&
+      >::type* = 0,
+      typename enable_if<
         !detail::is_work_dispatcher_required<
           typename decay<CompletionHandler>::type,
           Executor
@@ -181,7 +184,8 @@ public:
         !execution::is_executor<
           typename conditional<true, executor_type, CompletionHandler>::type
         >::value
-        &&
+      >::type* = 0,
+      typename enable_if<
         detail::is_work_dispatcher_required<
           typename decay<CompletionHandler>::type,
           Executor
diff --git a/asio/include/asio/prefer.hpp b/asio/include/asio/prefer.hpp
index b885ab46..9936f6d8 100644
--- a/asio/include/asio/prefer.hpp
+++ b/asio/include/asio/prefer.hpp
@@ -143,7 +143,9 @@ enum overload_type
   ill_formed
 };
 
-template <typename T, typename Properties, typename = void>
+template <typename T, typename Properties, typename = void,
+    typename = void, typename = void, typename = void,
+    typename = void, typename = void, typename = void>
 struct call_traits
 {
   ASIO_STATIC_CONSTEXPR(overload_type, overload = ill_formed);
@@ -154,16 +156,16 @@ struct call_traits
 template <typename T, typename Property>
 struct call_traits<T, void(Property),
   typename enable_if<
-    (
-      is_applicable_property<
-        typename decay<T>::type,
-        typename decay<Property>::type
-      >::value
-      &&
-      decay<Property>::type::is_preferable
-      &&
-      static_require<T, Property>::is_valid
-    )
+    is_applicable_property<
+      typename decay<T>::type,
+      typename decay<Property>::type
+    >::value
+  >::type,
+  typename enable_if<
+    decay<Property>::type::is_preferable
+  >::type,
+  typename enable_if<
+    static_require<T, Property>::is_valid
   >::type>
 {
   ASIO_STATIC_CONSTEXPR(overload_type, overload = identity);
@@ -179,18 +181,19 @@ struct call_traits<T, void(Property),
 template <typename T, typename Property>
 struct call_traits<T, void(Property),
   typename enable_if<
-    (
-      is_applicable_property<
-        typename decay<T>::type,
-        typename decay<Property>::type
-      >::value
-      &&
-      decay<Property>::type::is_preferable
-      &&
-      !static_require<T, Property>::is_valid
-      &&
-      require_member<T, Property>::is_valid
-    )
+    is_applicable_property<
+      typename decay<T>::type,
+      typename decay<Property>::type
+    >::value
+  >::type,
+  typename enable_if<
+    decay<Property>::type::is_preferable
+  >::type,
+  typename enable_if<
+    !static_require<T, Property>::is_valid
+  >::type,
+  typename enable_if<
+    require_member<T, Property>::is_valid
   >::type> :
   require_member<T, Property>
 {
@@ -200,20 +203,22 @@ struct call_traits<T, void(Property),
 template <typename T, typename Property>
 struct call_traits<T, void(Property),
   typename enable_if<
-    (
-      is_applicable_property<
-        typename decay<T>::type,
-        typename decay<Property>::type
-      >::value
-      &&
-      decay<Property>::type::is_preferable
-      &&
-      !static_require<T, Property>::is_valid
-      &&
-      !require_member<T, Property>::is_valid
-      &&
-      require_free<T, Property>::is_valid
-    )
+    is_applicable_property<
+      typename decay<T>::type,
+      typename decay<Property>::type
+    >::value
+  >::type,
+  typename enable_if<
+    decay<Property>::type::is_preferable
+  >::type,
+  typename enable_if<
+    !static_require<T, Property>::is_valid
+  >::type,
+  typename enable_if<
+    !require_member<T, Property>::is_valid
+  >::type,
+  typename enable_if<
+    require_free<T, Property>::is_valid
   >::type> :
   require_free<T, Property>
 {
@@ -223,22 +228,25 @@ struct call_traits<T, void(Property),
 template <typename T, typename Property>
 struct call_traits<T, void(Property),
   typename enable_if<
-    (
-      is_applicable_property<
-        typename decay<T>::type,
-        typename decay<Property>::type
-      >::value
-      &&
-      decay<Property>::type::is_preferable
-      &&
-      !static_require<T, Property>::is_valid
-      &&
-      !require_member<T, Property>::is_valid
-      &&
-      !require_free<T, Property>::is_valid
-      &&
-      prefer_member<T, Property>::is_valid
-    )
+    is_applicable_property<
+      typename decay<T>::type,
+      typename decay<Property>::type
+    >::value
+  >::type,
+  typename enable_if<
+    decay<Property>::type::is_preferable
+  >::type,
+  typename enable_if<
+    !static_require<T, Property>::is_valid
+  >::type,
+  typename enable_if<
+    !require_member<T, Property>::is_valid
+  >::type,
+  typename enable_if<
+    !require_free<T, Property>::is_valid
+  >::type,
+  typename enable_if<
+    prefer_member<T, Property>::is_valid
   >::type> :
   prefer_member<T, Property>
 {
@@ -248,24 +256,28 @@ struct call_traits<T, void(Property),
 template <typename T, typename Property>
 struct call_traits<T, void(Property),
   typename enable_if<
-    (
-      is_applicable_property<
-        typename decay<T>::type,
-        typename decay<Property>::type
-      >::value
-      &&
-      decay<Property>::type::is_preferable
-      &&
-      !static_require<T, Property>::is_valid
-      &&
-      !require_member<T, Property>::is_valid
-      &&
-      !require_free<T, Property>::is_valid
-      &&
-      !prefer_member<T, Property>::is_valid
-      &&
-      prefer_free<T, Property>::is_valid
-    )
+    is_applicable_property<
+      typename decay<T>::type,
+      typename decay<Property>::type
+    >::value
+  >::type,
+  typename enable_if<
+    decay<Property>::type::is_preferable
+  >::type,
+  typename enable_if<
+    !static_require<T, Property>::is_valid
+  >::type,
+  typename enable_if<
+    !require_member<T, Property>::is_valid
+  >::type,
+  typename enable_if<
+    !require_free<T, Property>::is_valid
+  >::type,
+  typename enable_if<
+    !prefer_member<T, Property>::is_valid
+  >::type,
+  typename enable_if<
+    prefer_free<T, Property>::is_valid
   >::type> :
   prefer_free<T, Property>
 {
@@ -275,24 +287,28 @@ struct call_traits<T, void(Property),
 template <typename T, typename Property>
 struct call_traits<T, void(Property),
   typename enable_if<
-    (
-      is_applicable_property<
-        typename decay<T>::type,
-        typename decay<Property>::type
-      >::value
-      &&
-      decay<Property>::type::is_preferable
-      &&
-      !static_require<T, Property>::is_valid
-      &&
-      !require_member<T, Property>::is_valid
-      &&
-      !require_free<T, Property>::is_valid
-      &&
-      !prefer_member<T, Property>::is_valid
-      &&
-      !prefer_free<T, Property>::is_valid
-    )
+    is_applicable_property<
+      typename decay<T>::type,
+      typename decay<Property>::type
+    >::value
+  >::type,
+  typename enable_if<
+    decay<Property>::type::is_preferable
+  >::type,
+  typename enable_if<
+    !static_require<T, Property>::is_valid
+  >::type,
+  typename enable_if<
+    !require_member<T, Property>::is_valid
+  >::type,
+  typename enable_if<
+    !require_free<T, Property>::is_valid
+  >::type,
+  typename enable_if<
+    !prefer_member<T, Property>::is_valid
+  >::type,
+  typename enable_if<
+    !prefer_free<T, Property>::is_valid
   >::type>
 {
   ASIO_STATIC_CONSTEXPR(overload_type, overload = identity);
@@ -309,7 +325,8 @@ template <typename T, typename P0, typename P1>
 struct call_traits<T, void(P0, P1),
   typename enable_if<
     call_traits<T, void(P0)>::overload != ill_formed
-    &&
+  >::type,
+  typename enable_if<
     call_traits<
       typename call_traits<T, void(P0)>::result_type,
       void(P1)
@@ -340,7 +357,8 @@ template <typename T, typename P0, typename P1, typename ASIO_ELLIPSIS PN>
 struct call_traits<T, void(P0, P1, PN ASIO_ELLIPSIS),
   typename enable_if<
     call_traits<T, void(P0)>::overload != ill_formed
-    &&
+  >::type,
+  typename enable_if<
     call_traits<
       typename call_traits<T, void(P0)>::result_type,
       void(P1, PN ASIO_ELLIPSIS)
diff --git a/asio/include/asio/query.hpp b/asio/include/asio/query.hpp
index 501b0198..39206bc2 100644
--- a/asio/include/asio/query.hpp
+++ b/asio/include/asio/query.hpp
@@ -118,7 +118,8 @@ enum overload_type
   ill_formed
 };
 
-template <typename T, typename Properties, typename = void>
+template <typename T, typename Properties, typename = void,
+    typename = void, typename = void, typename = void>
 struct call_traits
 {
   ASIO_STATIC_CONSTEXPR(overload_type, overload = ill_formed);
@@ -129,14 +130,13 @@ struct call_traits
 template <typename T, typename Property>
 struct call_traits<T, void(Property),
   typename enable_if<
-    (
-      is_applicable_property<
-        typename decay<T>::type,
-        typename decay<Property>::type
-      >::value
-      &&
-      static_query<T, Property>::is_valid
-    )
+    is_applicable_property<
+      typename decay<T>::type,
+      typename decay<Property>::type
+    >::value
+  >::type,
+  typename enable_if<
+    static_query<T, Property>::is_valid
   >::type> :
   static_query<T, Property>
 {
@@ -146,16 +146,16 @@ struct call_traits<T, void(Property),
 template <typename T, typename Property>
 struct call_traits<T, void(Property),
   typename enable_if<
-    (
-      is_applicable_property<
-        typename decay<T>::type,
-        typename decay<Property>::type
-      >::value
-      &&
-      !static_query<T, Property>::is_valid
-      &&
-      query_member<T, Property>::is_valid
-    )
+    is_applicable_property<
+      typename decay<T>::type,
+      typename decay<Property>::type
+    >::value
+  >::type,
+  typename enable_if<
+    !static_query<T, Property>::is_valid
+  >::type,
+  typename enable_if<
+    query_member<T, Property>::is_valid
   >::type> :
   query_member<T, Property>
 {
@@ -165,18 +165,19 @@ struct call_traits<T, void(Property),
 template <typename T, typename Property>
 struct call_traits<T, void(Property),
   typename enable_if<
-    (
-      is_applicable_property<
-        typename decay<T>::type,
-        typename decay<Property>::type
-      >::value
-      &&
-      !static_query<T, Property>::is_valid
-      &&
-      !query_member<T, Property>::is_valid
-      &&
-      query_free<T, Property>::is_valid
-    )
+    is_applicable_property<
+      typename decay<T>::type,
+      typename decay<Property>::type
+    >::value
+  >::type,
+  typename enable_if<
+    !static_query<T, Property>::is_valid
+  >::type,
+  typename enable_if<
+    !query_member<T, Property>::is_valid
+  >::type,
+  typename enable_if<
+    query_free<T, Property>::is_valid
   >::type> :
   query_free<T, Property>
 {
diff --git a/asio/include/asio/require.hpp b/asio/include/asio/require.hpp
index 6114e6b8..8e4f5867 100644
--- a/asio/include/asio/require.hpp
+++ b/asio/include/asio/require.hpp
@@ -127,7 +127,8 @@ enum overload_type
   ill_formed
 };
 
-template <typename T, typename Properties, typename = void>
+template <typename T, typename Properties, typename = void,
+    typename = void, typename = void, typename = void, typename = void>
 struct call_traits
 {
   ASIO_STATIC_CONSTEXPR(overload_type, overload = ill_formed);
@@ -138,16 +139,16 @@ struct call_traits
 template <typename T, typename Property>
 struct call_traits<T, void(Property),
   typename enable_if<
-    (
-      is_applicable_property<
-        typename decay<T>::type,
-        typename decay<Property>::type
-      >::value
-      &&
-      decay<Property>::type::is_requirable
-      &&
-      static_require<T, Property>::is_valid
-    )
+    is_applicable_property<
+      typename decay<T>::type,
+      typename decay<Property>::type
+    >::value
+  >::type,
+  typename enable_if<
+    decay<Property>::type::is_requirable
+  >::type,
+  typename enable_if<
+    static_require<T, Property>::is_valid
   >::type>
 {
   ASIO_STATIC_CONSTEXPR(overload_type, overload = identity);
@@ -163,18 +164,19 @@ struct call_traits<T, void(Property),
 template <typename T, typename Property>
 struct call_traits<T, void(Property),
   typename enable_if<
-    (
-      is_applicable_property<
-        typename decay<T>::type,
-        typename decay<Property>::type
-      >::value
-      &&
-      decay<Property>::type::is_requirable
-      &&
-      !static_require<T, Property>::is_valid
-      &&
-      require_member<T, Property>::is_valid
-    )
+    is_applicable_property<
+      typename decay<T>::type,
+      typename decay<Property>::type
+    >::value
+  >::type,
+  typename enable_if<
+    decay<Property>::type::is_requirable
+  >::type,
+  typename enable_if<
+    !static_require<T, Property>::is_valid
+  >::type,
+  typename enable_if<
+    require_member<T, Property>::is_valid
   >::type> :
   require_member<T, Property>
 {
@@ -184,20 +186,22 @@ struct call_traits<T, void(Property),
 template <typename T, typename Property>
 struct call_traits<T, void(Property),
   typename enable_if<
-    (
-      is_applicable_property<
-        typename decay<T>::type,
-        typename decay<Property>::type
-      >::value
-      &&
-      decay<Property>::type::is_requirable
-      &&
-      !static_require<T, Property>::is_valid
-      &&
-      !require_member<T, Property>::is_valid
-      &&
-      require_free<T, Property>::is_valid
-    )
+    is_applicable_property<
+      typename decay<T>::type,
+      typename decay<Property>::type
+    >::value
+  >::type,
+  typename enable_if<
+    decay<Property>::type::is_requirable
+  >::type,
+  typename enable_if<
+    !static_require<T, Property>::is_valid
+  >::type,
+  typename enable_if<
+    !require_member<T, Property>::is_valid
+  >::type,
+  typename enable_if<
+    require_free<T, Property>::is_valid
   >::type> :
   require_free<T, Property>
 {
@@ -208,7 +212,8 @@ template <typename T, typename P0, typename P1>
 struct call_traits<T, void(P0, P1),
   typename enable_if<
     call_traits<T, void(P0)>::overload != ill_formed
-    &&
+  >::type,
+  typename enable_if<
     call_traits<
       typename call_traits<T, void(P0)>::result_type,
       void(P1)
@@ -239,7 +244,8 @@ template <typename T, typename P0, typename P1, typename ASIO_ELLIPSIS PN>
 struct call_traits<T, void(P0, P1, PN ASIO_ELLIPSIS),
   typename enable_if<
     call_traits<T, void(P0)>::overload != ill_formed
-    &&
+  >::type,
+  typename enable_if<
     call_traits<
       typename call_traits<T, void(P0)>::result_type,
       void(P1, PN ASIO_ELLIPSIS)
diff --git a/asio/include/asio/require_concept.hpp b/asio/include/asio/require_concept.hpp
index d8785f6a..a2440377 100644
--- a/asio/include/asio/require_concept.hpp
+++ b/asio/include/asio/require_concept.hpp
@@ -125,7 +125,8 @@ enum overload_type
   ill_formed
 };
 
-template <typename T, typename Properties, typename = void>
+template <typename T, typename Properties, typename = void,
+    typename = void, typename = void, typename = void, typename = void>
 struct call_traits
 {
   ASIO_STATIC_CONSTEXPR(overload_type, overload = ill_formed);
@@ -136,16 +137,16 @@ struct call_traits
 template <typename T, typename Property>
 struct call_traits<T, void(Property),
   typename enable_if<
-    (
-      is_applicable_property<
-        typename decay<T>::type,
-        typename decay<Property>::type
-      >::value
-      &&
-      decay<Property>::type::is_requirable_concept
-      &&
-      static_require_concept<T, Property>::is_valid
-    )
+    is_applicable_property<
+      typename decay<T>::type,
+      typename decay<Property>::type
+    >::value
+  >::type,
+  typename enable_if<
+    decay<Property>::type::is_requirable_concept
+  >::type,
+  typename enable_if<
+    static_require_concept<T, Property>::is_valid
   >::type>
 {
   ASIO_STATIC_CONSTEXPR(overload_type, overload = identity);
@@ -156,18 +157,19 @@ struct call_traits<T, void(Property),
 template <typename T, typename Property>
 struct call_traits<T, void(Property),
   typename enable_if<
-    (
-      is_applicable_property<
-        typename decay<T>::type,
-        typename decay<Property>::type
-      >::value
-      &&
-      decay<Property>::type::is_requirable_concept
-      &&
-      !static_require_concept<T, Property>::is_valid
-      &&
-      require_concept_member<T, Property>::is_valid
-    )
+    is_applicable_property<
+      typename decay<T>::type,
+      typename decay<Property>::type
+    >::value
+  >::type,
+  typename enable_if<
+    decay<Property>::type::is_requirable_concept
+  >::type,
+  typename enable_if<
+    !static_require_concept<T, Property>::is_valid
+  >::type,
+  typename enable_if<
+    require_concept_member<T, Property>::is_valid
   >::type> :
   require_concept_member<T, Property>
 {
@@ -177,20 +179,22 @@ struct call_traits<T, void(Property),
 template <typename T, typename Property>
 struct call_traits<T, void(Property),
   typename enable_if<
-    (
-      is_applicable_property<
-        typename decay<T>::type,
-        typename decay<Property>::type
-      >::value
-      &&
-      decay<Property>::type::is_requirable_concept
-      &&
-      !static_require_concept<T, Property>::is_valid
-      &&
-      !require_concept_member<T, Property>::is_valid
-      &&
-      require_concept_free<T, Property>::is_valid
-    )
+    is_applicable_property<
+      typename decay<T>::type,
+      typename decay<Property>::type
+    >::value
+  >::type,
+  typename enable_if<
+    decay<Property>::type::is_requirable_concept
+  >::type,
+  typename enable_if<
+    !static_require_concept<T, Property>::is_valid
+  >::type,
+  typename enable_if<
+    !require_concept_member<T, Property>::is_valid
+  >::type,
+  typename enable_if<
+    require_concept_free<T, Property>::is_valid
   >::type> :
   require_concept_free<T, Property>
 {
diff --git a/asio/include/asio/ts/netfwd.hpp b/asio/include/asio/ts/netfwd.hpp
index 2642fca2..197e8d9d 100644
--- a/asio/include/asio/ts/netfwd.hpp
+++ b/asio/include/asio/ts/netfwd.hpp
@@ -45,7 +45,7 @@ class executor_binder;
 #if !defined(ASIO_EXECUTOR_WORK_GUARD_DECL)
 #define ASIO_EXECUTOR_WORK_GUARD_DECL
 
-template <typename Executor, typename = void>
+template <typename Executor, typename = void, typename = void>
 class executor_work_guard;
 
 #endif // !defined(ASIO_EXECUTOR_WORK_GUARD_DECL)