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

#pragma once

#include <iterator/concepts.h>

namespace iterator::recursive {
// Type deduction guides for constructing recursive iterators.
enum class recursion_type { END, THRU, ASSOC };

// Helpers for condensing type deductions
template <typename It> using value = decltype(std::begin(*std::declval<It>()));

template <typename V>
concept associative_value = std::is_const_v<typename V::first_type>;
template <typename V>
concept recursive_associative =
    associative_value<V> && Range<typename V::second_type>;

template <typename V>
constexpr auto typeclass =
    recursive_associative<V>
        ? recursion_type::ASSOC
        : (Range<V> ? recursion_type::THRU : recursion_type::END);
template <typename It>
constexpr auto iter_typeclass = typeclass<std::iter_value_t<It>>;
}