[Bug c++/33738] New: -Wtype-limits misses a warning when comparing enums

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

This was found on GCC 4.2.1.  In this test case, VRP quietly folds a comparison
between an enum type and a constant value that the enum type can never take.
With -Wtype-limits, this should give the warning:

comparison always false due to limited range of data type

extern void link_error (void);

enum Alpha {
 ZERO = 0, ONE, TWO, THREE
};

Alpha a2;

int m1 = -1;
int GetM1() {
 return m1;
}

int main() {
 a2 = static_cast<Alpha>(GetM1());
 if (a2 == -1) {                   <-- VRP should warn when folding this.
    link_error ();
 }
 return 0;
}

This is not warned by the front end because we promote -1 to the same type as
a2.  But during VRP, we *do* fold the predicate, so warning when -Wtype-limits
is given in this case would be a good QOI feature.

I have a patch in the works to make VRP warn when it linearizes this predicate.


-- 
           Summary: -Wtype-limits misses a warning when comparing enums
           Product: gcc
           Version: 4.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: dnovillo at gcc dot gnu dot org
        ReportedBy: dnovillo at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33738