I have come across the type "cv void" in the latest draft of the C++ standard (N4606) :
8.3.3 [dcl.mptr], paragraph 3
A pointer to member shall not point to a static member of a class (9.2.3), a member with reference type, or “cv void”.
3.9.1 [basic.fundamental], paragraph 9
A type cv void is an incomplete type that cannot be completed; such a type has an empty set of values...
"cv void" is not a real type. "cv" here is a shorthand for "cv-qualified", which means "may have a
const or a
volatile on it".
The passage means that
volatile void* and
const volatile void* are impossible in the context of pointers-to-members.