main.dart

class A<T extends dynamic> {
  Function(A<T>) go;
}

class B<T extends dynamic> extends A<T> {
  
}


void main() async {
  
  A b = B()..go = (foo) {
    print('test');
  };
  
  b.go(b);
  
  A a = A<String>()..go = (foo) {
    print('aa');
  };
  
  a.go(a);
  
  b = B<String>()..go = (foo) {
    print('test2');
  };
  
  b.go(b);
}