iterator: add index_type trait

iterator.hpp

@@ -229,6 +229,24 @@ namespace util {
     }
 
 
+    ///////////////////////////////////////////////////////////////////////////
+    /// a trait that stores a type suitable for indexing a container
+    template <typename, typename = std::void_t<>>
+    struct index_type { using type = std::size_t; };
+
+
+    //-------------------------------------------------------------------------
+    template <typename ContainerT>
+    struct index_type<
+        ContainerT,
+        std::void_t<
+            typename ContainerT::index_t
+        >
+    > {
+        using type = typename ContainerT::index_t;
+    };
+
+
     ///////////////////////////////////////////////////////////////////////////
     template <typename ContainerT>
     class indices {