Since a recent CVS update the assignment operator of an adcacency_list graph fails to compile under the IBM vacpp compiler (xlC). The simplest test case is: #include <boost/graph/adjacency_list.hpp> int main() { typedef boost::adjacency_list<boost::vecS,boost::vecS,boost::directedS> G; G g; g = G(); } which fails as follows: "/users/matroyer/boost/boost/optional/optional.hpp", line 353.66: 1540-0210 (S) "T" is not a base class of "pair<boost::detail::out_edge_iter<std::_Ptrit<boost::detail:: sep_<unsigned long,boost::no_property>,long,boost::detail::sep_<unsigned long,boost::no_property> *,boost::detail::sep_<unsigned long,boost::no_property> &,boost::detail::sep_<unsigned long,boost::no_property> *,boost::detail::sep_<unsigned long,boost::no_property> &>,unsigned long,boost::detail::edge_desc_impl<boost::directed_tag,unsigned long>,long>,boost::detail::out_edge_iter<std::_Ptrit<boo...". "/users/matroyer/boost/boost/optional/optional.hpp", line 353.10: 1540-0700 (I) The previous message was produced while processing "boost::optional_detail::optional_base<std::pair<boost::detail:: out_edge_iter<std::_Ptrit<boost::detail::sep_<unsigned long,boost::no_property>,long,boost::detail::sep_<unsigned long,boost::no_property> *,boost::detail::sep_<unsigned long,boost::no_property> &,boost::detail::sep_<unsigned long,boost::no_property> *,boost::detail::sep_<unsigned long,boost::no_property> &>,unsigned long,boost::detail::edge_desc_impl<boost::directed_tag,un...". "/users/matroyer/boost/boost/optional/optional.hpp", line 212.10: 1540-0700 (I) The previous message was produced while processing "boost::optional_detail::optional_base<std::pair<boost::detail:: out_edge_iter<std::_Ptrit<boost::detail::sep_<unsigned long,boost::no_property>,long,boost::detail::sep_<unsigned long,boost::no_property> *,boost::detail::sep_<unsigned long,boost::no_property> &,boost::detail::sep_<unsigned long,boost::no_property> *,boost::detail::sep_<unsigned long,boost::no_property> &>,unsigned long,boost::detail::edge_desc_impl<boost::directed_tag,un...". "/users/matroyer/boost/boost/graph/detail/adj_list_edge_iterator.hpp", line 38.11: 1540-0719 (I) The previous message was produced while processing the implicit member function "boost::detail::adj_list_edge_iterator<boost:: counting_iterator<unsigned long,boost::use_default,boost::use_default>,boost::detail:: out_edge_iter<std::_Ptrit<boost::detail::sep_<unsigned long,boost::no_property>,long,boost::detail::sep_<unsigned long,boost::no_property> *,boost::detail::sep_<unsigned long,boost::no_property> &,boost::detail::sep_<unsigned long,boost::no_property> *,boost::detail::sep_<unsign...". "/users/matroyer/boost/boost/tuple/detail/tuple_basic.hpp", line 578.10: 1540-0700 (I) The previous message was produced while processing "boost::tuples::tuple<boost::detail::adj_list_edge_iterator<boost:: counting_iterator<unsigned long,boost::use_default,boost::use_default>,boost::detail:: out_edge_iter<std::_Ptrit<boost::detail::sep_<unsigned long,boost::no_property>,long,boost::detail::sep_<unsigned long,boost::no_property> *,boost::detail::sep_<unsigned long,boost::no_property> &,boost::detail::sep_<unsigned long,boost::no_property> *,boost::detail::sep_<unsigned long,...". "/users/matroyer/boost/boost/graph/detail/adjacency_list.hpp", line 1943.19: 1540-0700 (I) The previous message was produced while processing "boost::vec_adj_list_impl<boost::adjacency_list<boost::vecS,boost:: vecS,boost::directedS,boost::no_property,boost::no_property,boost:: no_property,boost::listS>,struct boost::detail::adj_list_gen<class boost::adjacency_list<boost::vecS,boost::vecS,boost::directedS,boost:: no_property,boost::no_property,boost::no_property,boost::listS>,boost:: vecS,boost::vecS,boost::directedS,boost::no_property,boost:: no_property,boost::no_property,boost::...". "../../src/alps/lattice/unitcell.C", line 3.5: 1540-0700 (I) The previous message was produced while processing "main()" Can any of the BGL experts help? Matthias