Fix explicit instantiation regression

test/sp_explicit_inst_test.cpp

@@ -0,0 +1,23 @@
+//
+// Explicit instantiations are reported to exist in user code
+//
+// Copyright (c) 2014 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/shared_ptr.hpp>
+
+template class boost::shared_ptr< int >;
+
+struct X
+{
+};
+
+template class boost::shared_ptr< X >;
+
+int main()
+{
+}