https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79820 --- Comment #12 from Jonathan Wakely <redi at gcc dot gnu.org> --- I was wrong, and that code path is used by std::sync_with_stdio(false) when we reset cin, cout, cerr and clog to use stdio_filebuf stream buffers. So there's no need for a testcase.