diff --git a/asio/include/asio/detail/reactive_socket_service_base.hpp b/asio/include/asio/detail/reactive_socket_service_base.hpp
index 5e1fc9bf..9818cd90 100644
--- a/asio/include/asio/detail/reactive_socket_service_base.hpp
+++ b/asio/include/asio/detail/reactive_socket_service_base.hpp
@@ -197,7 +197,7 @@ public:
   template <typename ConstBufferSequence, typename Handler>
   void async_send(base_implementation_type& impl,
       const ConstBufferSequence& buffers,
-      socket_base::message_flags flags, Handler handler)
+      socket_base::message_flags flags, Handler& handler)
   {
     bool is_continuation =
       asio_handler_cont_helpers::is_continuation(handler);
@@ -221,7 +221,7 @@ public:
   // Start an asynchronous wait until data can be sent without blocking.
   template <typename Handler>
   void async_send(base_implementation_type& impl, const null_buffers&,
-      socket_base::message_flags, Handler handler)
+      socket_base::message_flags, Handler& handler)
   {
     bool is_continuation =
       asio_handler_cont_helpers::is_continuation(handler);
@@ -268,7 +268,7 @@ public:
   template <typename MutableBufferSequence, typename Handler>
   void async_receive(base_implementation_type& impl,
       const MutableBufferSequence& buffers,
-      socket_base::message_flags flags, Handler handler)
+      socket_base::message_flags flags, Handler& handler)
   {
     bool is_continuation =
       asio_handler_cont_helpers::is_continuation(handler);
@@ -296,7 +296,7 @@ public:
   // Wait until data can be received without blocking.
   template <typename Handler>
   void async_receive(base_implementation_type& impl, const null_buffers&,
-      socket_base::message_flags flags, Handler handler)
+      socket_base::message_flags flags, Handler& handler)
   {
     bool is_continuation =
       asio_handler_cont_helpers::is_continuation(handler);
@@ -353,7 +353,7 @@ public:
   template <typename MutableBufferSequence, typename Handler>
   void async_receive_with_flags(base_implementation_type& impl,
       const MutableBufferSequence& buffers, socket_base::message_flags in_flags,
-      socket_base::message_flags& out_flags, Handler handler)
+      socket_base::message_flags& out_flags, Handler& handler)
   {
     bool is_continuation =
       asio_handler_cont_helpers::is_continuation(handler);
@@ -380,7 +380,7 @@ public:
   template <typename Handler>
   void async_receive_with_flags(base_implementation_type& impl,
       const null_buffers&, socket_base::message_flags in_flags,
-      socket_base::message_flags& out_flags, Handler handler)
+      socket_base::message_flags& out_flags, Handler& handler)
   {
     bool is_continuation =
       asio_handler_cont_helpers::is_continuation(handler);
diff --git a/asio/include/asio/detail/win_iocp_socket_service_base.hpp b/asio/include/asio/detail/win_iocp_socket_service_base.hpp
index 1d023eec..3673b6db 100644
--- a/asio/include/asio/detail/win_iocp_socket_service_base.hpp
+++ b/asio/include/asio/detail/win_iocp_socket_service_base.hpp
@@ -214,7 +214,7 @@ public:
   template <typename ConstBufferSequence, typename Handler>
   void async_send(base_implementation_type& impl,
       const ConstBufferSequence& buffers,
-      socket_base::message_flags flags, Handler handler)
+      socket_base::message_flags flags, Handler& handler)
   {
     // Allocate and construct an operation to wrap the handler.
     typedef win_iocp_socket_send_op<ConstBufferSequence, Handler> op;
@@ -237,7 +237,7 @@ public:
   // Start an asynchronous wait until data can be sent without blocking.
   template <typename Handler>
   void async_send(base_implementation_type& impl, const null_buffers&,
-      socket_base::message_flags, Handler handler)
+      socket_base::message_flags, Handler& handler)
   {
     // Allocate and construct an operation to wrap the handler.
     typedef win_iocp_null_buffers_op<Handler> op;
@@ -281,7 +281,7 @@ public:
   template <typename MutableBufferSequence, typename Handler>
   void async_receive(base_implementation_type& impl,
       const MutableBufferSequence& buffers,
-      socket_base::message_flags flags, Handler handler)
+      socket_base::message_flags flags, Handler& handler)
   {
     // Allocate and construct an operation to wrap the handler.
     typedef win_iocp_socket_recv_op<MutableBufferSequence, Handler> op;
@@ -304,7 +304,7 @@ public:
   // Wait until data can be received without blocking.
   template <typename Handler>
   void async_receive(base_implementation_type& impl, const null_buffers&,
-      socket_base::message_flags flags, Handler handler)
+      socket_base::message_flags flags, Handler& handler)
   {
     // Allocate and construct an operation to wrap the handler.
     typedef win_iocp_null_buffers_op<Handler> op;
@@ -355,7 +355,7 @@ public:
   template <typename MutableBufferSequence, typename Handler>
   void async_receive_with_flags(base_implementation_type& impl,
       const MutableBufferSequence& buffers, socket_base::message_flags in_flags,
-      socket_base::message_flags& out_flags, Handler handler)
+      socket_base::message_flags& out_flags, Handler& handler)
   {
     // Allocate and construct an operation to wrap the handler.
     typedef win_iocp_socket_recvmsg_op<MutableBufferSequence, Handler> op;
@@ -378,7 +378,7 @@ public:
   template <typename Handler>
   void async_receive_with_flags(base_implementation_type& impl,
       const null_buffers&, socket_base::message_flags in_flags,
-      socket_base::message_flags& out_flags, Handler handler)
+      socket_base::message_flags& out_flags, Handler& handler)
   {
     // Allocate and construct an operation to wrap the handler.
     typedef win_iocp_null_buffers_op<Handler> op;
diff --git a/asio/include/asio/detail/win_object_handle_service.hpp b/asio/include/asio/detail/win_object_handle_service.hpp
index f0a1f442..051f536a 100644
--- a/asio/include/asio/detail/win_object_handle_service.hpp
+++ b/asio/include/asio/detail/win_object_handle_service.hpp
@@ -129,7 +129,7 @@ public:
 
   /// Start an asynchronous wait.
   template <typename Handler>
-  void async_wait(implementation_type& impl, Handler handler)
+  void async_wait(implementation_type& impl, Handler& handler)
   {
     // Allocate and construct an operation to wrap the handler.
     typedef wait_handler<Handler> op;