no type named ‘pointer’ in struct std::iterator_traits<...>

Here are the fragments of my linked_list template :

template < class T , class Allocator = std::allocator< T > >
class linked_list final
{/*... */
    enum constantness { CONST , MUTABLE } ;
    template< constantness is_const > 
    class iterator_base : public std::iterator< std::bidirectional_iterator_tag , node< T > , std::ptrdiff_t , 
                                typename std::conditional< is_const == MUTABLE , T * , const T * >::type , 
                                typename std::conditional< is_const == MUTABLE , T& , const T& >::type >
    {   /* ... */
        typename std::iterator_traits< iterator_base >::pointer operator -> () const { return &( to_obj_node( current_node_ ) -> object() ) ; }
        typename std::iterator_traits< iterator_base >::reference operator * () const { return to_obj_node( current_node_ ) -> object() ; }
        /* ... */
    } ;

    public :  /* types : */
        /* ... */
        using iterator = iterator_base< MUTABLE > ;
        using const_iterator = iterator_base< CONST > ;     
/* I  */using difference_type = typename std::iterator_traits< iterator >::difference_type ;
/* II */using difference_type = std::ptrdiff_t ; 
        /* ... */
} ;


int main( void )
{
    linked_list< int > list{ 1 , 2 , 5 , 7 , 8 , 4 } ;
    auto it = list.begin() ;
    * it = 9 ;
    return 0 ;
}

When I leave line that's marked first uncommented, the code doesn't compile.

g++5.4 :

list2.cxx:105:66: error: no type named ‘pointer’ in ‘struct std::iterator_traits<linked_list<int>::iterator_base<(linked_list<int, std::allocator<int> >::constantness)1u> >’
          typename std::iterator_traits< iterator_base >::pointer operator -> () const { return &( to_obj_node( current_node_ ) -> object() ) ; }

icpc :

list.cxx(105): error: incomplete type is not allowed typename std::iterator_traits< iterator_base >::pointer operator -> () const { return &( to_obj_node( current_node_ ) -> object() ) ; }

With second line all compiles fine.