List.Hpp Template <typename T, typename = void> struct is_iterator : std::false_type {

Question

List.Hpp Template <typename T, typename = void> struct is_iterator : std::false_type {

};
template <typename T>
struct is_iterator<
T, std::void_t<
decltype(*std::declval<T>()),
decltype(++std::declval<T>())>> :
std::true_type {
};

template <
typename InputIt,
typename std::enable_if<is_iterator<InputIt>::value>
>
list(InputIt from, InputIt to, const Allocator& = Allocator());

list.cpp
template <typename T, typename Allocator>
template <
typename InputIt,
typename std::enable_if<is_iterator<InputIt>::value>
>
jm::list<T, Allocator>::list(InputIt from, InputIt to, const Allocator&) {

}
Выдает undefined reference, че не так?