[Bug c++/24745] New: unpleasant warning for "if (NULL)"

[mec at google dot com]

C++ programs with "if (NULL)" generate an unpleasant warning even with default
compiler options.  Code with "if (NULL)" comes from of macro expansion or
auto-generated code.

Warnings are a matter of taste so I won't ask "make all the warnings match my
taste", but I do petition you to revisit which of these lines should generate
warnings and apply your own taste.

This patch looks like the last time these warnings were modified:

http://gcc.gnu.org/ml/gcc-patches/2005-01/msg02230.html
Mark Mitchell, C++ PATCH: PR 19457 a PR 19349

Test program:

int foo() {
  if (__null) return -1;
  if (!__null) ; else return -1;
  if (__null == __null) ; else return -1;
  if (__null != __null) return -1;
  if (__null == __null) ; else return -1;
  if (__null != 0) ; else return -1;
  return 0;
}

int main() {
  return foo() == 0;
}

Warnings shown:

hollerith:~/null-arithmetic$ /home/mec/gcc-4.0.2/install/bin/gcc -c
null-arithmetic.cc
null-arithmetic.cc: In function 'int foo()':
null-arithmetic.cc:2: warning: converting to non-pointer type 'bool' from NULL
null-arithmetic.cc:3: warning: converting to non-pointer type 'bool' from NULL
null-arithmetic.cc:4: warning: NULL used in arithmetic
null-arithmetic.cc:5: warning: NULL used in arithmetic
null-arithmetic.cc:6: warning: NULL used in arithmetic
null-arithmetic.cc:7: warning: NULL used in arithmetic

hollerith:~/null-arithmetic$ /home/mec/gcc-4.1-20051105/install/bin/gcc -c
null-arithmetic.cc
null-arithmetic.cc: In function 'int foo()':
null-arithmetic.cc:2: warning: converting to non-pointer type 'bool' from NULL
null-arithmetic.cc:3: warning: converting to non-pointer type 'bool' from NULL
null-arithmetic.cc:4: warning: NULL used in arithmetic
null-arithmetic.cc:5: warning: NULL used in arithmetic
null-arithmetic.cc:6: warning: NULL used in arithmetic
null-arithmetic.cc:7: warning: NULL used in arithmetic


-- 
           Summary: unpleasant warning for "if (NULL)"
           Product: gcc
           Version: 4.0.2
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: mec at google dot com
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24745