Recursive iterator is a forward_iterator. This is guaranteed by end_aware_iterator as well, but should be made explicit.

recursive_iterator.hpp

@@ -99,7 +99,7 @@ namespace iterator { namespace detail {
     using reference = typename super::reference;
     using pointer = typename super::pointer;
     using difference_type = typename super::difference_type;
-    using iterator_category = typename super::iterator_category;
+    using iterator_category = std::forward_iterator_tag;
   public:
     recursive_iterator_layer() = default;
     recursive_iterator_layer(layer v) : recursive_iterator_layer() {
@@ -194,7 +194,7 @@ namespace iterator { namespace detail {
                                               std::declval<next_reference>()));
     using pointer = void;
     using difference_type = typename super::difference_type;
-    using iterator_category = typename super::iterator_category;
+    using iterator_category = std::forward_iterator_tag;
   public:
     flatten_iterator_layer() = default;
     flatten_iterator_layer(layer v) : flatten_iterator_layer() {