If it is not known what is in the p variable, then the variable is indeterminate; it has exactly the same status as a variable that has not been initialized. In this case, the behavior upon accessing the variable should be undefined.

I agree with making accesses of NaN-valued variables undefined behavior, so that not comparing equal is then a possible consequence of undefined behavior.

I don't agree with defining the unequal comparison as the required behavior. To define the behavior is tantamount to the recognition that a NaN is something: an object. A variable can have a defined value which is that something, and that value must obey the Law of Identity.