Class as_charn_impl

include/boost/parser/detail/text/transcode_view.hpp:388–411

    namespace detail {
        template<template<class> class View, format Format>
        struct as_charn_impl : stl_interfaces::range_adaptor_closure<as_charn_impl<View, Format>>
        {
#if BOOST_PARSER_DETAIL_TEXT_USE_CONCEPTS
            template<class R>
            requires (std::ranges::viewable_range<R> &&
                      std::ranges::input_range<R> &&
                      std::convertible_to<std::ranges::range_reference_t<R>, format_to_type_t<Format>>)
#else
            template<class R>
#endif
            [[nodiscard]] constexpr auto operator()(R && r) const
            {
                using T = remove_cv_ref_t<R>;
                if constexpr (detail::is_empty_view<T>) {
#if BOOST_PARSER_DETAIL_TEXT_USE_CONCEPTS
                    return std::ranges::empty_view<format_to_type_t<Format>>{};
#else
                    return 42; // Never gonna happen.
#endif
                } else {
                    return View(std::forward<R>(r));
                }
            }
        };

        template<class T>
        constexpr bool is_charn_view = false;