https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103626
--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> --- I don't think we need to add the #error to every header. For a start, users never include the bits/* headers directly, and if they do, it's their problem. So we only need to mark the standard headers as hosted-only. And if we add it to <iosfwd>, that already covers a large number of them, because they include that.
