[Bug preprocessor/94127] New: #pragma system_header marks unrelated files as system headers

[aleksey.covacevice at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94127

            Bug ID: 94127
           Summary: #pragma system_header marks unrelated files as system
                    headers
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: preprocessor
          Assignee: unassigned at gcc dot gnu.org
          Reporter: aleksey.covacevice at gmail dot com
  Target Milestone: ---

When compiling `x.c` as below:

    /* x.c */
    #include <sys.h>
    #include <usr.h>


    /* sys.h */
    #pragma GCC system_header
    #include "sys2.h"

    /* usr.h */
    #include "other.h"

    /* sys2.h empty */
    /* other.h empty */

file "other.h" gets marked as a system header, as per -fdebug-cpp:

   
{P:./other.h;F:./usr.h;L:1;C:0;S:1;M:0x7f323a64b180;E:0,LOC:260288,R:260288}
                                   ^^^

This affects dependency generation with -MMD.

If `sys.h` includes `<sys2.h>` (in angle brackets), or includes nothing at all,
this does not happen. Neither does it happen if `x.c` includes either file with
quotes.

Nothing in the documentation suggests this behavior is intended.