Reading the standard, in 5.2.7/6 we find that unless the target is an unambiguous base of the source, source must be a polymorphic type. Then in 10.3/1
Virtual functions support dynamic binding and objectoriented
programming. A class that declares or inherits a virtual function is
called a polymorphic class.
In other words the standard doesn’t seem to say anything about your question. In this case, the standard doesn’t allow for a compiler to turn off RTTI so for each compiler you need to check its documentation to see what would happen. Based on this reading, I think this is a compiler question, not a C++ language question as the tag indicates.
Alternately you can avoid the problem completely by just using static_cast when you know it’s sufficient.