What does pointer_traits provide for types that are neither X<A, T…> nor provide a member typedef element_type?

Question

What is the result of following? Is it illformed, undefined behavior or welldefined and wellformed?

struct A {};
std::pointer_traits<A> x;

The reason I ask is because I want to know for curiousity, and because I want to know whether an arbitrary type is a pointer. With that I also mean to include shared_ptr and friends. I wondered whether there is a type trait (predicate) for that or, if not, whether I can use pointer_traits and detect whether element_type is declared or not.

Answer 1

It says it is ill-formed as from 20.6.3p1 since it doesn't have element_type and is not a class template instantiation

typedef see below element_type;

Type: Ptr::element_type if such a type exists; otherwise, T if Ptr is a class template instantiation of the form SomePointer<T, Args> , where Args is zero or more type arguments; otherwise, the specialization is ill-formed .