Recursive de-referencable nil_

include/utl/meta/integral.h

@@ -33,8 +33,15 @@
 namespace utl {
 namespace meta {
 
-   //! Empty type
-   struct nil_ { };
+   /*!
+    * Empty type
+    * utl::meta's nil type is not pure nil. It's a recursive "de-referencable nil.
+    * Each time someone applies ::type to it, he gets back nil_. This way we can prevent
+    * a lot of compilation errors in a wrong meta:: handling.
+    */
+   struct nil_ {
+      using type = nil_;
+   };
 
    //! Type alias for \p _Tp::type. Used to extract return type of metafunctions
    template <typename _Tp>