[Bug c/54787] New: Inconsistent -Wtype-limits warning for different-sized bitfields

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

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54787

             Bug #: 54787
           Summary: Inconsistent -Wtype-limits warning for different-sized
                    bitfields
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: hp@gcc.gnu.org
              Host: x86_64-unknown-linux-gnu


For the attached test-case, GCC warns inconsistently: it warns when the
bit-field is 8 bits, but not 7 nor 9 bits.  The c++ front-end warns
consistently for all three lines.  Observed with trunk r191987, the 4.7 branch
(r191916, post 4.7.2), the 4.6 branch (r191812, post4.6.3) as well as local
imports from the 4.3-branch and the host compiler, "gcc-4.4.3-4.fc12.x86_64"). 
Other targets include crisv32-axis-linux-gnu and mipsisa32r2el-linux-gnu.

Example output (note absence of warnings for lines 28 and 30):
gcc -Wtype-limits -O2 -c range.c
/n/pp_slask/hp/range.c: In function 'main':
/n/pp_slask/hp/range.c:29: warning: comparison is always true due to limited
range of data type

There's a related need to turn off the warning per-expression, to avoid the
need to turn it off per-file (by adding "-Wno-error=type-limits" to
compilations using the common idiom of -Werror together with warnings that
include -Wtype-limits).