fixup! WIP [skip ci]

include/boost/gil/step_iterator.hpp

@@ -69,18 +69,19 @@ class step_iterator_adaptor
 
     // It is really common for iterator adaptors to have a base() member
     // function that returns the adapted iterator.
-    int* base() const { return it_; }
+    Iterator base() const { return it_; }
 
 protected:
     Iterator it_;
-    SFn _step_fn;    
+    SFn _step_fn;
+
+    constexpr Iterator& base_reference() noexcept { return it_; }
+    constexpr Iterator const& base_reference() const noexcept { return it_; }
+
 private:
     // Provide access to these private members.
     friend boost::stl_interfaces::access;
 
-    constexpr int*& base_reference() noexcept { return it_; }
-    constexpr int* base_reference() const noexcept { return it_; }
-
     //void increment() { _step_fn.advance(this->base_reference(),1); }
     //void decrement() { _step_fn.advance(this->base_reference(),-1); }
     constexpr void advance(base_difference_type d) { _step_fn.advance(this->base_reference(),d); }