SFINAE out the converting constructor of arg<I> to avoid hard errors with is_convertible on g++ 4.8/4.9

test/arg_copy_fail.cpp

@@ -0,0 +1,19 @@
+//
+//  arg_copy_fail.cpp - arg<1> to arg<2>
+//
+//  Copyright 2016 Peter Dimov
+//
+//  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
+//
+
+#include <boost/bind/arg.hpp>
+
+//
+
+int main()
+{
+    boost::arg<1> a1(( boost::arg<2>() ));
+    (void)a1;
+}