[boost] [xpressive] Broken in release if explicit operator bool is available

Hi, With the recent change to Boost.SmartPtr that adds support for explicit operator bool() Boost.Xpressive no longer compiles. The relevant error message is: ./boost/xpressive/detail/utility/tracking_ptr.hpp:440:73: error: no type named ‘unspecified_bool_type’ in ‘class boost::intrusive_ptr<boost::xpressive::detail::regex_impl<const char*>

’

Could this be fixed?