Using pure to create immutable

[Jesse Phillips]

The discussion on Reddit brought to my attention that pure functions can return 
and assign to an immutable.

http://www.reddit.com/r/programming/comments/knn5p/thoughts_on_immutability_in_d/c2lsgek

I am trying to modify the example request to make use of this, but have failed.

http://www.reddit.com/r/programming/comments/knn5p/thoughts_on_immutability_in_d/c2lrfpm

test.d(4): Error: cannot implicitly convert expression (makeFromArray([1,2,3]))
of type test.List!(int).List to immutable(List)

Is this a bug? I can't identify where this issue would lie (works with 
inheritance and templating).

void main() {
   immutable a = makeFromArray([1,2,3]);
}
private abstract class List(T) {
   abstract bool isEmpty () const;
   abstract T head () const;
   abstract const(List!T) tail () const;


}
private final class Cons(T): List!T {
   immutable T head_;
   Cons!T tail_; // not immutable here for a reason

   this(T h, Cons!T t) { head_ = h; tail_ = t; } 

   override bool isEmpty() const { return false; }
   override T head () const { return head_; }
   override const(Cons!T) tail () const { return tail_; }
}

List!T makeFromArray(T)(T[] array) pure {
   if (array.length == 0) { return null; }

   auto result = new Cons!T(array[0], null);
   auto end = result;

   for (int i = 1; i < array.length; ++i) {
      end.tail_ = new Cons!T(array[i], null);
      end = end.tail_;
   }

   return result;
}