Hello.

You have not been very clear about how you want your new warning to work
but given the example you provided:

   class base {
       virtual void foo() {}
   };

   class derived: public base {
       [non_overriden] base::foo; //will warn when called
   };

I have to ask if

   class derived: public base {
       [[deprecated("Warn when called")]] void foo() { base::foo(); }
   };

does what you want?