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

test/arg_copy_test.cpp

@@ -0,0 +1,34 @@
+//
+//  arg_copy_test.cpp - copying a custom placeholder _1 to arg<1>
+//
+//  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/is_placeholder.hpp>
+#include <boost/bind/arg.hpp>
+
+//
+
+template<int I> struct ph
+{
+};
+
+namespace boost
+{
+
+template<int I> struct is_placeholder< ::ph<I> >
+{
+    enum _vt { value = I };
+};
+
+} // namespace boost
+
+int main()
+{
+    boost::arg<1> a1 = ph<1>();
+    (void)a1;
+}