https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84998
--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> --- Also: #define _GLIBCXX_DEBUG #include <vector> int main() { std::vector<bool> b; std::hash<std::vector<bool>> h; return h(b); } And a number of other debug mode failures related to access. I suspect FE improvements to lookup have made some of our friend declarations stop working as intended.