I've encountered a serious behavioral breakage for code using flat_xxx in conjunction with ordered_unique_range and a non-default comparator in 1.59.0b1. Here is a small repro: #include <initializer_list> #include <algorithm> #include <functional> #include <iterator> #include <iostream> #include <boost/container/flat_set.hpp> int main() { namespace bcont = boost::container; auto const p = [](auto const& desc, auto const& s) { std::cout << desc << ": "; std::copy( s.cbegin(), s.cend(), std::ostream_iterator<int>(std::cout, " ") ); std::cout << '\n'; }; auto const r = {4, 3, 2}; bcont::flat_set<int, std::greater<int>> const s{3, 2, 1}; p("orig set", s); auto s1{s}; s1.insert(r.begin(), r.end()); p("without\tordered_unique_range (expected output)", s1); auto s2{s}; s2.insert(bcont::ordered_unique_range, r.begin(), r.end()); p("with\tordered_unique_range (erroneous output)", s2); } I've worked around this locally by rerouting the ordered_unique_range codepath to the default codepath, but this seems like a showstopper for 1.59.0 as the code's behavior has broken entirely silently. Regards, Adam Merz