foo.cc: In member function ‘PolyLinkNode& PolyLinkNode::operator=(PolyLinkNode*)’:
foo.cc:30:24: warning: reference to local variable ‘res’ returned [-Wreturn-local-addr]
   30 |                 return res;
      |                        ^~~
foo.cc:28:30: note: declared here
   28 |                 PolyLinkNode res;
      |                              ^~~
foo.cc: In member function ‘PolyLinkNode& PolyLinkNode::Add(PolyLinkNode&)’:
foo.cc:119:24: warning: reference to local variable ‘res’ returned [-Wreturn-local-addr]
  119 |                 return res;
      |                        ^~~
foo.cc:69:30: note: declared here
   69 |                 PolyLinkNode res;
      |                              ^~~
foo.cc: In member function ‘PolyLinkNode& PolyLinkNode::Mul(PolyLinkNode&)’:
foo.cc:145:24: warning: reference to local variable ‘ans’ returned [-Wreturn-local-addr]
  145 |                 return ans;
      |                        ^~~
foo.cc:135:30: note: declared here
  135 |                 PolyLinkNode ans;
      |                              ^~~