Add test for taking placeholders by const& (phoenix::bind takes its arguments by const&)

test/Jamfile.v2

@@ -51,4 +51,5 @@ test-suite "bind"
       [ run bind_fwd_test.cpp ]
       [ run bind_fwd2_test.cpp ]
       [ run bind_no_placeholders_test.cpp ]
+      [ run placeholder_const_ref_test.cpp ]
     ;

test/placeholder_const_ref_test.cpp

@@ -0,0 +1,37 @@
+//
+//  placeholder_const_ref_test.cpp - forming a const& to _1
+//
+//  Copyright 2015 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/placeholders.hpp>
+#include <boost/is_placeholder.hpp>
+#include <boost/core/lightweight_test.hpp>
+
+//
+
+template<class T> void test( T const &, int i )
+{
+    BOOST_TEST_EQ( boost::is_placeholder<T>::value, i );
+}
+
+int main()
+{
+    using namespace boost::placeholders;
+
+    test( _1, 1 );
+    test( _2, 2 );
+    test( _3, 3 );
+    test( _4, 4 );
+    test( _5, 5 );
+    test( _6, 6 );
+    test( _7, 7 );
+    test( _8, 8 );
+    test( _9, 9 );
+
+    return boost::report_errors();
+}