As of 212638:

    gcc/c-family/ChangeLog:
        * c-ppoutput.c (struct print::prev_was_system_token): New data
        member.
        (init_pp_output): Initialize it.
        (maybe_print_line_1, maybe_print_line, print_line_1, print_line)
        (do_line_change): Return a flag saying if a line marker was
        emitted or not.
        (scan_translation_unit): Detect if the system-ness of the token we
        are about to emit is different from the one of the previously
        emitted token.  If so, emit a line marker.  Avoid emitting useless
        adjacent line markers.  Avoid emitting line markers for tokens
        originating from the expansion of built-in macros.
        (scan_translation_unit_directives_only): Adjust.

by dodji

gcc fails to build ncurses. Basically the preprocessor ends up generating invalid C-code.

I'll try to reduce a testcase soon, but in the meantime this appears on x86, arm and aarch64 so should be easily reproducible