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

test/placeholder_std_bind_test.cpp

@@ -0,0 +1,46 @@
+//
+//  placeholder_std_bind_test.cpp - std::bind with Boost's _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/config.hpp>
+
+#if defined( BOOST_NO_CXX11_HDR_FUNCTIONAL )
+
+int main()
+{
+}
+
+#else
+
+#include <boost/bind.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <functional>
+
+namespace std
+{
+
+template<int N> struct is_placeholder< boost::arg<N> >: public integral_constant<int, N> {};
+
+} // namespace std
+
+int foo( int i )
+{
+    return i;
+}
+
+int main()
+{
+    BOOST_TEST_EQ( std::bind( foo, _1 )( 1 ), 1 );
+    BOOST_TEST_EQ( std::bind( foo, _2 )( 1, 2 ), 2 );
+    BOOST_TEST_EQ( std::bind( foo, _3 )( 1, 2, 3 ), 3 );
+
+    return boost::report_errors();
+}
+
+#endif