lib/58912: ctype(3) abuse detection fails for variable references

[campbell+netbsd%mumble.net@localhost]

>Number:         58912
>Category:       lib
>Synopsis:       ctype(3) abuse detection fails for variable references
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    lib-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Wed Dec 18 01:55:00 +0000 2024
>Originator:     Taylor R Campbell
>Release:        current, 10, 9?
>Organization:
The isnetbsd Foundation
>Environment:
>Description:
The ctype(3) abuse detection trick with -Wchar-subscripts works when the input is an array element, pointer dereference, or function call, but not when it's a variable reference:

$ cat foo.c
#include <ctype.h>
extern char x;
int f(void) { return isspace(x); }
$ cat bar.c
#include <ctype.h>
extern char x[];
int f(void) { return isspace(x[0]); }
$ cc -c -Wall -Werror foo.c
$ cc -c -Wall -Werror bar.c
In file included from /usr/include/ctype.h:100,
                 from bar.c:1:
bar.c: In function 'f':
bar.c:3:31: error: array subscript has type 'char' [-Werror=char-subscripts]
    3 | int f(void) { return isspace(x[0]); }
      |                               ^
cc1: all warnings being treated as errors

Expected both cc invocations to report warning.
>How-To-Repeat:

>Fix:
Yes, please!

Putting

#pragma GCC diagnostic push
#pragma GCC diagnostic warning "-Wsystem-headers"
...
#pragma GCC diagnostic pop

around the definitions in sys/ctype_inline.h may work but may also have side effects I haven't thought of yet.