[Bug c/110047] New: RFE: Add a warning for use of bare "unsigned" (possibly under -Wimplicit-int?)

["egallager at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110047

            Bug ID: 110047
           Summary: RFE: Add a warning for use of bare "unsigned"
                    (possibly under -Wimplicit-int?)
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: enhancement
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: egallager at gcc dot gnu.org
            Blocks: 87403
  Target Milestone: ---

When I was first learning C, one thing that confused me was how you can just
use plain "unsigned" as a type, without specifying the length (long, short,
int, etc.). Thus, I thought that casting to unsigned would just change the sign
like a call to abs(), without realizing that there was an implicit "int"
involved. I made a testcase:

$ cat bare_unsigned.c
#include <limits.h>

unsigned var; /* debatable */

unsigned long foo(void)
{
        long variable = LONG_MAX;
        unsigned long uvariable = (unsigned)variable; /* warn here */
        return uvariable;
}
$

The one where I added the "debatable" comment is debatable because I actually
see a lot of declarations in that form pretty often, and it's probably not very
harmful in that case, but the case with the cast, where it says "warn here", is
probably more deserving of a warning, as there's a change of size involved. It
might make sense to include this under -Wimplicit-int, or maybe create a new
warning -Wbare-unsigned for it?


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87403
[Bug 87403] [Meta-bug] Issues that suggest a new warning