sfinae.h

template<typename T, typename Index = std::size_t, typename = void>
struct has_subscript_operator : std::false_type {};

template<typename T, typename Index>
struct has_subscript_operator<T, Index, std::void_t<decltype(std::declval<T>()[std::declval<Index>()])>> : std::true_type {};

template<typename T, typename Index = int>
using has_subscript_operator_v = typename has_subscript_operator<T, Index>::value;

template<typename T>
using iterator_cat = typename std::iterator_traits<T>::iterator_category;

template<typename T, typename = void>
struct is_iterator : std::false_type {};

template<typename T>
struct is_iterator<T, std::void_t<iterator_cat<T>>> : std::true_type {};