Support Concepts for completion token params

include/boost/beast/_experimental/http/icy_stream.hpp

@@ -224,7 +224,7 @@ public:
     */
     template<
         class MutableBufferSequence,
-        class ReadHandler =
+        BOOST_BEAST_ASYNC_TPARAM2 ReadHandler =
             net::default_completion_token_t<executor_type>
     >
     BOOST_BEAST_ASYNC_RESULT2(ReadHandler)
@@ -306,7 +306,7 @@ public:
     */
     template<
         class ConstBufferSequence,
-        class WriteHandler =
+        BOOST_BEAST_ASYNC_TPARAM2 WriteHandler =
             net::default_completion_token_t<executor_type>
     >
     BOOST_BEAST_ASYNC_RESULT2(WriteHandler)

include/boost/beast/_experimental/test/stream.hpp

@@ -477,7 +477,7 @@ public:
     */
     template<
         class MutableBufferSequence,
-        class ReadHandler>
+        BOOST_BEAST_ASYNC_TPARAM2 ReadHandler>
     BOOST_BEAST_ASYNC_RESULT2(ReadHandler)
     async_read_some(
         MutableBufferSequence const& buffers,
@@ -555,7 +555,7 @@ public:
     */
     template<
         class ConstBufferSequence,
-        class WriteHandler>
+        BOOST_BEAST_ASYNC_TPARAM2 WriteHandler>
     BOOST_BEAST_ASYNC_RESULT2(WriteHandler)
     async_write_some(
         ConstBufferSequence const& buffers,