https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97537
--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> --- Oh sorry, I misread your example. The second #include "header3.h" is indeed reached. The header is skipped due to the optimization described at https://gcc.gnu.org/onlinedocs/cpp/Once-Only-Headers.html so the second include is skipped and the file doesn't get reopened. I suppose that's the cause: because the file is skipped, it isn't considered "used" again. So I think I agree with your bug report, the output should not be affected by the optimization, and header3.h should be considered "used" again, even if it isn't actually opened again.
