[Bug c++/71904] New: comparing this to NULL should give a warning by default (this == NULL)

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71904

            Bug ID: 71904
           Summary: comparing this to NULL should give a warning by
                    default (this == NULL)
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: manu at gcc dot gnu.org
  Target Milestone: ---

Given the grief brought by the null-pointer check elimination for this, it
would be good if C++ diagnosed the simplest cases, so that users are not
shocked when their code starts segfaulting:
https://gcc.gnu.org/bugzilla/PR71892


void bar();
class A {
  void test(void) { if (this == 0) bar();}
};

Clang:

rog.cc:3:25: warning: 'this' pointer cannot be null in well-defined C++ code;
comparison may be assumed to always evaluate to false
[-Wtautological-undefined-compare]
  void test(void) { if (this == 0) bar();}
                        ^~~~    ~