cpp-useful-iterators/include/iterator/detail/traits.h

#pragma once

#define _val(type) std::declval<type>()
#define exists(expr) void_t<decltype(expr)>

namespace iterator::detail {
template <typename> using void_t = void;

template <typename T, typename = void> struct reference_helper {
  using type = decltype(*std::declval<T>());
};
template <typename T>
struct reference_helper<T, void_t<typename T::reference>> {
  using type = typename T::reference;
};

template <typename T, typename = void> struct value_type_helper {
  using reference = typename reference_helper<T>::type;
  using type = std::remove_cv_t<std::remove_reference_t<reference>>;
};
template <typename T>
struct value_type_helper<T, void_t<typename T::value_type>> {
  using type = typename T::value_type;
};

template <typename T> using value_type = typename value_type_helper<T>::type;
template <typename T> using reference = typename reference_helper<T>::type;
}