[Bug c/64518] New: Warning about comparison between signed and unsigned can be useless in some cases

[patrick.pelissier at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64518

            Bug ID: 64518
           Summary: Warning about comparison between signed and unsigned
                    can be useless in some cases
           Product: gcc
           Version: 4.9.1
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: patrick.pelissier at gmail dot com

For the following function:

int f(int x) { return ( x >= 0 && x < sizeof(int)); } 


I get the following warning with gcc -Wsign-compare -c f.c :
f.c: In function 'f':
f.c:1:37: warning: comparison between signed and unsigned integer expressions
[-Wsign-compare]
 int f(int x) { return ( x >= 0 && x < sizeof(int)); } 
                                     ^


However, as x has been checked as positive due to the first comparison, I don't
see how the comparison between x (signed) and sizeof(int) (unsigned) can be
wrong.

The proposed enhancement is to improve the warning to handle this case properly
without adding a (useless) cast in the user code like this:

int f(int x) { return ( x >= 0 && (unsigned int) x < sizeof(int)); }