View source for cpp/utility/format/format kind

===Possible implementation===
{{eq fun
|1=
namespace detail
{
    template< typename >
    constexpr bool is_pair_or_tuple_2 = false;
    
    template< typename T, typename U >
    constexpr bool is_pair_or_tuple_2<std::pair<T, U>> = true;
    
    template< typename T, typename U >
    constexpr bool is_pair_or_tuple_2<std::tuple<T, U>> = true;
    
    template < typename T >
        requires std::is_reference_v<T> {{!!}} std::is_const_v<T>
    constexpr bool is_pair_or_tuple_2<T> =
        is_pair_or_tuple_2<std::remove_cvref_t<T>>;
}

template< class R >
constexpr range_format format_kind = []
{
    static_assert(false, "instantiating a primary template is not allowed");
    return range_format::disabled;
}();

template< ranges::input_range R >
    requires std::same_as<R, std::remove_cvref_t<R>>
constexpr range_format format_kind<R> = []
{
    if constexpr (std::same_as<std::remove_cvref_t<std::ranges::range_reference_t<R>>, R>)
        return range_format::disabled;
    else if constexpr (requires { typename R::key_type; })
    {
        if constexpr (requires { typename R::mapped_type; } &&
                      detail::is_pair_or_tuple_2<std::ranges::range_reference_t<R>>)
            return range_format::map;
        else
            return range_format::set;
    }
    else
        return range_format::sequence;
}();
}}