[C++11] instantiated member class inside the template class

A member class inside a template class should/should not being instantiated?
quoted sample code:

// empty list:
template<typename ... Args>
struct List {
    struct head {static_assert(sizeof...(Args) != 0, "Attempt to access the head of an empty list."); };
    struct tail {static_assert(sizeof...(Args) != 0, "Attempt to access the tail of an empty list."); };
};

// non-empty list:
template<typename Head, typename... Tail>
struct List<Head, Tail...> {
    typedef Head head;
    typedef List<tail ...> tail;
};

1. (Clause 14.7.1p1) Unless a class template specialization has been explicitly instantiated (14.7.2) or explicitly specialized (14.7.3), the class template specialization is implicitly instantiated when the specialization is referenced in a context that requires a completely-defined object type or when the completeness of the class type affects the semantics of the program. The implicit instantiation of a class template specialization causes the implicit instantiation of the declarations, but not of the definitions ... of the class member functions, member classes, [...].
2. (Clause 14.7.1p11) An implementation shall not implicitly instantiate ... a member class...of a class template that does not require instantiation.
3. (Clause 14.6p8) If no valid specialization can be generated for a template, and that template is not instantiated, the template is ill-formed, no diagnostic required.