::contains

#include <algorithm>
#include <functional>
#include <iterator>
#include <ranges>

namespace impl {
    struct contains_fn {
        template<std::input_iterator I, std::sentinel_for<I> S,
                 typename T, typename Proj = std::identity>
            requires std::indirect_binary_predicate<
                std::ranges::equal_to,
                std::projected<I, Proj>,
                const T*
            >
        constexpr bool operator()(I first, S last, const T& value, Proj proj = {}) const {
            return std::ranges::find(first, last, value, std::ref(proj)) != last;
        }

        template<std::ranges::input_range R, typename T, typename Proj = std::identity>
            requires std::indirect_binary_predicate<
                std::ranges::equal_to,
                std::projected<std::ranges::iterator_t<R>, Proj>,
                const T*
            >
        constexpr bool operator()(R&& r, const T& value, Proj proj = {}) const {
            return (*this)(std::ranges::begin(r), std::ranges::end(r), value, std::ref(proj));
        }
    };
}

inline constexpr impl::contains_fn contains{};