https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85269
--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> --- (In reply to Jonathan Wakely from comment #2) > Also, we'd need to be careful so that we don't warn about referring to > std::ostream without <ostream>, because <iosfwd> exists specifically to make > that possible. Also, <iostream> is now guaranteed to include <ostream>, but wasn't in previous versions of the standard. According to the standard, std::char_traits is defined by <string>, but in practice it has to be defined by <ios> and <string_view> as well, so it would be unhelpful to warn about using it without <string>. Similarly, std::allocator is defined in <memory>, but has to be defined by most containers (and other headers) so it would be unhelpful to warn about using std::allocator without <memory>. Making this warning actually useful might be a lot of work.
