The Polygon library has a minor C++ standards-conformance issue w.r.t. unqualified name lookup in templates for 'gtlsort' operator. In particular, the header <boost/polygon/polygon.hpp> includes a number of source files that make unqualified calls to 'gtlsort'. 'gtlsort' itself is not defined until much later, when <boost/polygon/detail/scan_arbitrary.hpp> pulls in <boost/detail/polygon_sort_adaptor.hpp>. At least in the cases used in the Boost.Polygon test, 'gtlsort' isn't found by ADL at instantiation time, either. Clang detects this conformance problem, which is described here: http://clang.llvm.org/compatibility.html#dep_lookup The solution is to provide forward declarations of the two 'gtlsort' overloads in a header that all users of 'gtlsort' depend on. For example, introducing a forward header containing namespace boost { namespace polygon { template <typename iter_type> void gtlsort(iter_type _b_, iter_type _e_); template <typename iter_type, typename pred_type> void gtlsort(iter_type _b_, iter_type _e_, const pred_type& _pred_); } } and including it before the users of 'gtlsort' would work fine. - Doug