[Bug c++/77513] -Wzero-as-null-pointer-constant vs 0, nullptr, NULL and __null

["petschy at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77513

--- Comment #5 from petschy at gmail dot com ---
Some more details, hope this helps. Preprocessed one of the oddly behaving
files with 5.4.1, 6.2.1 and 7.0.0, then tried to compile each preprocessed file
with each compiler version. 5.4.1 warned for all preprocessed files, 6.2.1 and
7.0.0 didn't warn for any of them. This doesn't mean that 6.2.1 and 7.0.0 is
not affected, just for this particular file they didn't warn. 

Diffing the files revealed that there is no difference in the user code, only
in the system headers included, eg <string> comes from
/usr/local/include/c++/5.4.1/string for 5.4.1, etc.

All of the problematic lines are assignments of the form
  var = NULL;
or
  var1 = var2 = NULL;

ALL of the NULL assignments in the file are reported.
NONE of the == or != tests are reported.

Eg: this code warns:

AbstractImage::AbstractImage()
{
 pixels=
# 124 "common/src/AbstractImage.cpp" 3 4
       __null
# 124 "common/src/AbstractImage.cpp"
           ;
 channels_n=0;
 mode=255;
 width=-1;
 height=-1;
}

But if I replace 'pixels =' with 'bool b = pixels ==' above, the warning
disappears, which is strange, I think.

The flags are the same for the != and == tests, ie 
# <n> "common/src/AbstractImage.cpp" 3 4
is everywhere.