[boost] Question about is_enum

From: Kevin S. Van Horn (Kevin.VanHorn_at_[hidden])
Date: 2002-10-14 09:47:39

I've been looking over some of the type traits stuff, and I had a question
about the implementation of is_enum.  Here is the code in question:

template <typename T> struct is_enum
{
private:
   typedef ::boost::add_reference<T> ar_t;
   typedef typename ar_t::type r_type;
       
   BOOST_STATIC_CONSTANT(bool, selector =
      (::boost::type_traits::ice_or<
           ::boost::is_arithmetic<T>::value
         , ::boost::is_reference<T>::value
       // We MUST do this on conforming compilers in order to
       // correctly deduce that noncopyable types are not enums (dwa
       // 2002/04/15)...
         , ::boost::is_class<T>::value
      >::value));
    typedef ::boost::detail::is_enum_helper<selector> se_t;
    typedef typename se_t::template type<r_type> helper;
public:
    BOOST_STATIC_CONSTANT(bool, value = helper::value);
};

I'm quite mystified by the comment about ORing is_class<T>::value into 
selector.  How is the fact that a class C is noncopyable going to make
is_enum_helper<false>::type<r_type>::value true?  I tried to look up the 
mentioned message in the mailing list archives, and it didn't shed any 
light.