std::ranges::concat_view<Views...>::end

constexpr auto end()
    requires (!(/*simple-view*/<Views> && ...));

constexpr auto end() const
    requires (ranges::range<const Views> && ...) &&
                 /*concatable*/<const Views...>;

1) Эквивалентно

constexprautoN=sizeof...(Views);
ifconstexpr(ranges::common_range<Views...[N-1]>)
    return iterator <false>(this,std::in_place_index<N-1>,
                           ranges::end(std::get<N-1>(views_ )));
else
    returnstd::default_sentinel;

2) Эквивалентно

constexprautoN=sizeof...(Views);
ifconstexpr(ranges::common_range<constViews...[N-1]>)
    return iterator <true>(this,std::in_place_index<N-1>,
                          ranges::end(std::get<N-1>(views_ )));
else
    returnstd::default_sentinel;

#include <concepts>
#include <iterator>
#include <print>
#include <ranges>

int main()
{
    static constexpr int p[]{37, 42, 69};
    static constexpr auto q = {19937, 1729};
    constexpr auto cat = std::ranges::views::concat(p, q);
    static_assert(not std::same_as<std::default_sentinel_t, decltype(cat.end())>);
    static_assert(cat.end()[-1] == 1729);
    std::println("{}", cat);
}

[37, 42, 69, 19937, 1729]