Virtual Inheritance: Error: no unique final overrider

Question

I know virtual inheritance is covered here before and before asking this question, I went through the detail of the virtual inheritance and went through the details of a sim

Answer 1

Using the scope resolution operator to specify that you want to call base::fun doesn't make the error go away because the program would be ill-formed even with an empty main(). You simply are not allowed to have a situation in which a virtual function has more than one final overrider in any derived class that exists in your program.

Informally, just because trying to call p->fun() would be ambiguous, the program is ill-formed even if you don't do it.

Note: This is in contrast to the situation you have with overloaded functions, in which potential ambiguity is allowed---perhaps even unavoidable---as long as you avoid a call that actually would be ambiguous. Why are the rules different? Basically it is because even constructing an object of type der3 cannot be done in a sensible way---which version of fun should the vtable point to?