[iter] Adjust source_of/sink_of

src/hb-iter.hh

@@ -277,7 +277,7 @@ struct hb_is_source_of
 {
   private:
   template <typename Iter2 = Iter,
-	    hb_enable_if (hb_is_convertible (typename Iter2::item_t, const Item &))>
+	    hb_enable_if (hb_is_convertible (typename Iter2::item_t, hb_add_lvalue_reference<hb_add_const<Item>>))>
   static hb_true_type impl (hb_priority<2>);
   template <typename Iter2 = Iter>
   static auto impl (hb_priority<1>) -> decltype (hb_declval (Iter2) >> hb_declval (Item &), hb_true_type ());
@@ -293,7 +293,7 @@ struct hb_is_sink_of
 {
   private:
   template <typename Iter2 = Iter,
-	    hb_enable_if (hb_is_convertible (typename Iter2::item_t, Item &))>
+	    hb_enable_if (hb_is_convertible (typename Iter2::item_t, hb_add_lvalue_reference<Item>))>
   static hb_true_type impl (hb_priority<2>);
   template <typename Iter2 = Iter>
   static auto impl (hb_priority<1>) -> decltype (hb_declval (Iter2) << hb_declval (Item), hb_true_type ());