https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125996
--- Comment #1 from Drea Pinski <pinskia at gcc dot gnu.org> ---
/* Also emit a target for the include name, so for #include
<iostream> you'd make iostream.c++-header-unit, regardless of
what actual directory iostream lives in. We reconstruct the
include name by skipping the directory where we found it. */
auto *dir = _cpp_get_file_dir (pfile->main_file);
gcc_assert (!strncmp (d->module_name, dir->name, dir->len));
