decorator constructor is constrained

include/boost/beast/websocket/detail/decorator.hpp

@@ -25,6 +25,27 @@ namespace beast {
 namespace websocket {
 namespace detail {
 
+// VFALCO NOTE: When this is two traits, one for
+//              request and one for response,
+//              Visual Studio 2015 fails.
+
+template<class T, class U, class = void>
+struct can_invoke_with : std::false_type
+{
+};
+
+template<class T, class U>
+struct can_invoke_with<T, U, boost::void_t<decltype(
+    std::declval<T&>()(std::declval<U&>()))>>
+    : std::true_type
+{
+};
+
+template<class T>
+using is_decorator = std::integral_constant<bool,
+    can_invoke_with<T, request_type>::value ||
+    can_invoke_with<T, response_type>::value>;
+
 class decorator
 {
     friend class decorator_test;

include/boost/beast/websocket/stream_base.hpp

@@ -14,6 +14,7 @@
 #include <boost/beast/websocket/detail/decorator.hpp>
 #include <boost/beast/core/role.hpp>
 #include <chrono>
+#include <type_traits>
 
 namespace boost {
 namespace beast {
@@ -65,7 +66,13 @@ struct stream_base
             @param f An invocable function object. Ownership of
             the function object is transferred by decay-copy.
         */
-        template<class Decorator>
+        template<class Decorator
+#ifndef BOOST_BEAST_DOXYGEN
+            ,class = typename std::enable_if<
+                detail::is_decorator<
+                    Decorator>::value>::type
+#endif
+        >
         decorator(Decorator&& f)
             : d_(std::forward<Decorator>(f))
         {