[Bug c++/96003] [11 Regression] Maybe a false positive for -Werror=nonnull

[slyfox at inbox dot ru]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96003

--- Comment #7 from Sergei Trofimovich <slyfox at inbox dot ru> ---
Similar example from xmms2 project, dynamic_cast<> version:

  #include <typeinfo>

  // main loop interface
  struct I {
    virtual void run();
  };

  struct M : public I {
    virtual void run();
    void setup_M();
  };

  struct Client
  {
   // In real code Client() always initializes mainloop_ with non-NULL 'M*'
pointer;
   I* mainloop_;
   Client();
   void run_client();

   void connect() {
      if (mainloop_ && typeid(mainloop_) == typeid(M)) {
          dynamic_cast<M*>(mainloop_)->setup_M( );
      }
   }
  };

$ LANG=C x86_64-pc-linux-gnu-g++-11.0.0 -c bug.cpp -o a.o -Werror=nonnull
bug.cpp: In member function 'void Client::connect()':
bug.cpp:22:49: error: 'this' pointer null [-Werror=nonnull]
   22 |           dynamic_cast<M*>(mainloop_)->setup_M( );
      |                                                 ^
bug.cpp:10:10: note: in a call to non-static member function 'void
M::setup_M()'
   10 |     void setup_M();
      |          ^~~~~~~
cc1plus: some warnings being treated as errors

Original code is from
https://github.com/xmms2/xmms2-devel/blob/dedc33d7408e140bce714c2c3eb5bcc793f1af6c/src/clients/lib/xmmsclient%2B%2B/client.cpp#L85