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From: "mikpe at it dot uu.se" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/50012] New: C++ front end misses -Wsign-compare warnings when extraneous parentheses are present
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50012

           Summary: C++ front end misses -Wsign-compare warnings when
                    extraneous parentheses are present
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: mikpe@it.uu.se


In this test case two similar functions compare an unsigned int with a signed
int, differing only in whether the unsigned sub-expression has an extra pair of
parentheses around it or not.  The C FE correctly issues a -Wsign-compare
diagnostic for both functions, but the C++ FE fails to do so for the function
with the extra parentheses.

> cat bug.c
int foo(unsigned int *a, int b)
{
    return (*a) <= b;
}

int bar(unsigned int *a, int b)
{
    return *a <= b;
}
> gcc/xgcc -Bgcc/ -O2 -Wsign-compare -S bug.c
bug.c: In function 'foo':
bug.c:3:17: warning: comparison between signed and unsigned integer expressions
[-Wsign-compare]
bug.c: In function 'bar':
bug.c:8:15: warning: comparison between signed and unsigned integer expressions
[-Wsign-compare]
> gcc/g++ -Bgcc/ -O2 -Wsign-compare -S /tmp/bug.c
/tmp/bug.c: In function 'int bar(unsigned int*, int)':
/tmp/bug.c:8:18: warning: comparison between signed and unsigned integer
expressions [-Wsign-compare]

This difference exists in 4.7-20110806, 4.6.1, and 4.5.3, but not in 4.4.6.

The test case is derived from ipa-inline-analysis.c, specifically Jan Hubicka's
recent change in r177484.  That change introduced a signed/unsigned comparison,
but due to this C++ FE bug and the recent "build stage2 etc with g++ by
default" change, the error went undetected.  That is, until people started
using --disable-build-poststage1-with-cxx to reduce bootstrap times, where it
ultimately breaks bootstrap, see PR50005.