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

include/boost/bind/arg.hpp

@@ -21,20 +21,27 @@
 
 #include <boost/config.hpp>
 #include <boost/is_placeholder.hpp>
-#include <boost/static_assert.hpp>
 
 namespace boost
 {
 
+template< int I, int J > struct _arg_eq
+{
+};
+
+template< int I > struct _arg_eq< I, I>
+{
+    typedef void type;
+};
+
 template< int I > struct arg
 {
     BOOST_CONSTEXPR arg()
     {
     }
 
-    template< class T > BOOST_CONSTEXPR arg( T const & /* t */ )
+    template< class T > BOOST_CONSTEXPR arg( T const & /* t */, typename _arg_eq< I, is_placeholder<T>::value >::type * = 0 )
     {
-        BOOST_STATIC_ASSERT( I == is_placeholder<T>::value );
     }
 };
 

test/Jamfile.v2

@@ -56,4 +56,7 @@ test-suite "bind"
       [ run bind_type_test.cpp ]
       [ run bind_unique_ptr_test.cpp ]
       [ run bind_nested_rv_test.cpp ]
+      [ compile arg_copy_test.cpp ]
+      [ compile-fail arg_copy_fail.cpp ]
+      [ run placeholder_std_bind_test.cpp ]
     ;

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;
+}

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;
+}

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