https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99692
Andrew Pinski <pinskia at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- Component|c++ |libstdc++ --- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> --- Right the problem is here (and not with the compiler itself if there is a problem): /home/ubuntu/upstream-gcc/include/c++/11.0.1/ostream:747:46: error: no match for ‘operator<<’ (operand types are ‘std::basic_ostream<char>’ and ‘const std::vector<int>’) 746 | __void_t<decltype(declval<_Ostream&>() | ~~~~~~~~~~~~~~~~~~~~ 747 | << declval<const _Tp&>())>> | ^~~~~~~~~~~~~~~~~~~~~~~~ But by definition at this point operator << does not exist and is not considered as it is not found via ADL because both std::basic_ostream<char> and std::vector<int> are in the std namespace and the global namespace is not considered. /home/ubuntu/upstream-gcc/include/c++/11.0.1/ostream:773:5: required by substitution of ‘template<class _Ostream, class _Tp> typename std::enable_if<std::__and_<std::__not_<std::is_lvalue_reference<_Tp> >, std::__is_convertible_to_basic_ostream<_Ostream>, std::__is_insertable<typename std::__is_convertible_to_basic_ostream<_Tp>::__ostream_type, const _Tp&, void> >::value, typename std::__is_convertible_to_basic_ostream<_Tp>::__ostream_type>::type std::operator<<(_Ostream&&, const _Tp&) [with _Ostream = CustomStream; _Tp = std::vector<int>]’ t78.cc:21:41: required from here template<typename _Ostream, typename _Tp> inline typename enable_if<__and_<__not_<is_lvalue_reference<_Ostream>>, __is_convertible_to_basic_ostream<_Ostream>, __is_insertable< __rvalue_ostream_type<_Ostream>, const _Tp&>>::value, __rvalue_ostream_type<_Ostream>>::type operator<<(_Ostream&& __os, const _Tp& __x) It is the __is_insertable in the above. So it might be the wrong type being used to check __is_insertable ....