[Bug c/99950] New: Option -Wchar-subscripts leads to wrong fixes

[Bug c/99950] New: Option -Wchar-subscripts leads to wrong fixes

[roland.illig at gmx dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99950

            Bug ID: 99950
           Summary: Option -Wchar-subscripts leads to wrong fixes
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: roland.illig at gmx dot de
  Target Milestone: ---

When code calls the functions from <ctype.h> with an expression of type 'char',
GCC and Clang warn:

> array subscript has type char

Since the function prototypes expect an 'int', inexperienced programmers cast
the argument to 'int', which silences the warning:

https://github.com/search?q=%22isspace+int%22&type=code

More experienced programmers who heard something about 'unsigned' cast the
argument to 'unsigned int', which silences the warning as well:

https://github.com/search?q=%22isspace+unsigned+int%22&type=code

To get these programmers to fix their code properly, GCC should provide more
guidance in this area.  As a quick prototype I implemented a check for these
wrong "fixes" in NetBSD's lint, which results in warnings like these:

warning: argument to 'function from <ctype.h>' must be cast to 'unsigned char',
not to 'int' [342]

The implementation is here, including a test:

https://github.com/NetBSD/src/blob/fdd2c09ca474/usr.bin/xlint/lint1/ckctype.c
https://github.com/NetBSD/src/blob/fdd2c09ca474/tests/usr.bin/xlint/lint1/msg_341.c
https://github.com/NetBSD/src/blob/fdd2c09ca474/tests/usr.bin/xlint/lint1/msg_341.exp

Maybe GCC can implement a similar check to prevent these wrong fixes in the
future.

[Bug c/99950] Option -Wchar-subscripts leads to wrong fixes

[egallager at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99950

Eric Gallager <egallager at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=94182
           Keywords|                            |diagnostic
                 CC|                            |egallager at gcc dot gnu.org

--- Comment #1 from Eric Gallager <egallager at gcc dot gnu.org> ---
related to bug 94182

[Bug c/99950] Option -Wchar-subscripts leads to wrong fixes

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99950

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
                 CC|                            |msebor at gcc dot gnu.org
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=78155
   Last reconfirmed|                            |2021-04-07
           Severity|normal                      |enhancement
             Status|UNCONFIRMED                 |NEW

--- Comment #2 from Martin Sebor <msebor at gcc dot gnu.org> ---
Adding a note after the warning suggesting to cast the argument to unsigned
char would help.  Enabling the warning also when the char argument is cast to a
wider type might also be worth considering.  See also pr78155 for a related
request to enable the warning for calls to the character classification
built-ins.

$ cat pr99950.c && gcc -O2 -S -Wall -Wchar-subscripts pr99950.c
extern int a[255];

int f (char c)
{
  return a[c];                   // -Wchar-subscripts
}

int g (char c)
{
  return a[(int)c];              // same bug, no warning
}
pr99950.c: In function ‘f’:
pr99950.c:5:11: warning: array subscript has type ‘char’ [-Wchar-subscripts]
    5 |   return a[c];                   // -Wchar-subscripts
      |           ^

[Bug c/99950] Option -Wchar-subscripts leads to wrong fixes when used with functions from <ctype.h>

[egallager at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99950

Eric Gallager <egallager at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|Option -Wchar-subscripts    |Option -Wchar-subscripts
                   |leads to wrong fixes        |leads to wrong fixes when
                   |                            |used with functions from
                   |                            |<ctype.h>

--- Comment #3 from Eric Gallager <egallager at gcc dot gnu.org> ---
retitling for searchability purposes