[Bug c/78568] New: Wtype-limits warning regression

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78568

            Bug ID: 78568
           Summary: Wtype-limits warning regression
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sirl at gcc dot gnu.org
  Target Milestone: ---

Created attachment 40179
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=40179&action=edit
testcase

The attached testcase produces warnings with gcc-4.4.7 on RHEL6, but somewhere
along the way it regressed. I've tested 4.6, 4.8, 4.9, 5, 6 and trunk, none of
them warns anymore, but they all still happily optimize the comparisons away.

gcc-4.4.7 produced:
$ gcc -O2 -Wtype-limits -c Wtype-limits.c 
Wtype-limits.c: In function 'testa':
Wtype-limits.c:4: warning: comparison is always true due to limited range of
data type
Wtype-limits.c: In function 'testb':
Wtype-limits.c:12: warning: comparison is always true due to limited range of
data type
Wtype-limits.c: In function 'testc':
Wtype-limits.c:20: warning: comparison is always true due to limited range of
data type