Function if_

include/boost/parser/parser.hpp:8631–8634

        `if_(pred)[p]`, `p` is only applied if `pred` is true. */
    template<typename Predicate>
    constexpr auto if_(Predicate pred) noexcept
    {
        return if_directive<Predicate>{pred};
    }

    namespace detail {
        template<typename SwitchValue, typename Value>