[Bug c++/99251] New: Strange -Wnonnull warning behaviour with dynamic_cast

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99251

            Bug ID: 99251
           Summary: Strange -Wnonnull warning behaviour with dynamic_cast
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sirl at gcc dot gnu.org
  Target Milestone: ---

With this testcase:

class cl1 {
  virtual void m();
};
class cl2 : public cl1 {
public:
  int g();
  int h();
  int i();
};
class cl3 {
  cl1 *p;
  int g();
  int h();
  int i();
};
int cl3::g() {
  if (!p)
    return 0;
  cl2 *x = dynamic_cast<cl2 *>(p);
  return x->g();
}
int cl3::h() {
  if (!p)
    return 0;
  return (dynamic_cast<cl2 *>(p))->h();
}
int cl3::i() {
  if (!p)
    return 0;
  return dynamic_cast<cl2 *>(p)->i();
}

compiled with g++-trunk@r11-7356 -Wnonnull this warning is issued:

testcase.cpp: In member function 'int cl3::i()':
testcase.cpp:30:36: warning: 'this' pointer is null [-Wnonnull]
   30 |   return dynamic_cast<cl2 *>(p)->i();
      |                                    ^
testcase.cpp:8:7: note: in a call to non-static member function 'int cl2::i()'
    8 |   int i();
      |       ^

I believe there should be no warning for cl3::i() like with g++-10.2.