[Bug c++/45918] New: Lack of warning on meaningless unsigned to zero comparison

[Bug c++/45918] New: Lack of warning on meaningless unsigned to zero comparison

[yuri at tsoft dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45918

           Summary: Lack of warning on meaningless unsigned to zero
                    comparison
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: yuri@tsoft.com


In this case idx<NUM-1 is always false.
gcc should issue an error: comparison is always false due to the value range
limitations.

--- testcase ---
typedef unsigned size_t;

template<size_t NUM>
struct G {
  void fn();
};

template<size_t NUM>
void G<NUM>::fn() {
    for (size_t idx = 0; idx<NUM-1; idx++) {
    }
}

template struct G<1>;

[Bug c++/45918] Lack of warning on meaningless unsigned to zero comparison

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45918

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |paolo.carlini at oracle dot
                   |                            |com

--- Comment #1 from Paolo Carlini <paolo.carlini at oracle dot com> 2010-10-06 23:10:35 UTC ---
I suspect part of this is actually intended, we used to have stronger warnings
at some point in this area, but then people noticed that unavoidably too many
false positives are often produced with templates... Do you have any idea if we
ever warned for this, say, in 4.0,x, 4.1.x?

[Bug c++/45918] Lack of warning on meaningless unsigned to zero comparison

[yuri at tsoft dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45918

--- Comment #2 from Yuri <yuri at tsoft dot com> 2010-10-06 23:16:21 UTC ---
No warning in 4.2.1 for example.

[Bug c++/45918] Lack of warning on meaningless unsigned to zero comparison

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45918

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |manu at gcc dot gnu.org

--- Comment #3 from Paolo Carlini <paolo.carlini at oracle dot com> 2010-10-06 23:18:20 UTC ---
Let's add Manuel in CC...

[Bug c++/45918] Lack of warning on meaningless unsigned to zero comparison

[manu at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45918

Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |WAITING
   Last reconfirmed|                            |2010.10.07 00:45:06
     Ever Confirmed|0                           |1

--- Comment #4 from Manuel López-Ibáñez <manu at gcc dot gnu.org> 2010-10-07 00:45:06 UTC ---
manuel@gcc11:~$ test/164733M/build/gcc/cc1plus -Wall -Wextra test.C
 void G<NUM>::fn() void G<NUM>::fn() [with unsigned int NUM = 1u]
test.C:14:17:   instantiated from here
test.C:10:21: warning: comparison of unsigned expression < 0 is always false
[-Wtype-limits]

manuel@gcc11:~$ test/164733M/build/gcc/xgcc -v
Using built-in specs.
COLLECT_GCC=test/164733M/build/gcc/xgcc
Target: x86_64-unknown-linux-gnu
Configured with: /home/manuel/test/src/configure
--prefix=/home/manuel/test/./164733M/install --enable-languages=all,ada
--enable-decimal-float --with-mpc=/opt/cfarm/mpc-0.8
--with-mpfr=/opt/cfarm/mpfr-2.4.1/
Thread model: posix
gcc version 4.6.0 20100929 (experimental) [trunk revision 146344] (GCC)

[Bug c++/45918] Lack of warning on meaningless unsigned to zero comparison

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45918

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|WAITING                     |RESOLVED
         Resolution|                            |WORKSFORME

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> 2011-03-29 17:23:48 UTC ---
works for me, just use -Wtype-limits or -Wextra