Fix counting iterator.

Author: gogagum

include/boost/iterator/counting_iterator.hpp

@@ -171,6 +171,7 @@ class counting_iterator :
 
 public:
     using reference = typename super_t::reference;
+    using value_type = typename super_t::value_type;
     using difference_type = typename super_t::difference_type;
 
     counting_iterator() = default;
@@ -183,6 +184,11 @@ class counting_iterator :
     {
     }
 
+    value_type operator[](difference_type n) const {
+        auto ret_iter = *this;
+        return *(ret_iter + n);
+    }
+
 private:
     reference dereference() const
     {

include/boost/iterator/iterator_facade.hpp

@@ -641,6 +641,7 @@ class iterator_facade_base< Derived, Value, CategoryOrTraversal, Reference, Diff
 
 public:
     using reference = typename base_type::reference;
+    using value_type = typename base_type::value_type;
     using difference_type = typename base_type::difference_type;
 
 public: