https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102028
--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> --- (In reply to Lars Maier from comment #0) > Reason: the result of operator<< with std::stringstream and A const& is just > an std::ostream& No it isn't, https://cplusplus.github.io/LWG/issue1203 changed that.
