view: use SFINAE to diable casts that can't be aligned

view.hpp

@@ -393,10 +393,34 @@ namespace cruft {
 
 
         ///////////////////////////////////////////////////////////////////////
+        /// Explicitly cast to a view with different iterator types.
+        ///
+        /// The source and destination iterator types must be:
+        ///   * pointers
+        ///   * alignable
+        ///
+        /// It is undefined behaviour to cast with begin and/or end iterators
+        /// that do not have natural alignment. Instrumented builds _may_
+        /// provide diagnostics or assertions in this case.
+        ///
+        /// \tparam ValueT   The new iterator type
+        /// \return          A view that uses the new iterator type
         template <
             typename ValueT,
             typename = std::enable_if_t<
-                std::is_pointer_v<BeginT> && std::is_pointer_v<ValueT>
+                // We can only convert views that use pointer iterators
+                std::is_pointer_v<BeginT>   &&
+                std::is_pointer_v<EndT>     &&
+                std::is_same_v<BeginT,EndT> &&
+                std::is_pointer_v<ValueT>   &&
+                // The values they point to must allow for alignment in one
+                // direction or another.
+                (
+                    sizeof (typename std::iterator_traits<BeginT>::value_type) %
+                    sizeof (typename std::iterator_traits<ValueT>::value_type) == 0 ||
+                    sizeof (typename std::iterator_traits<ValueT>::value_type) %
+                    sizeof (typename std::iterator_traits<BeginT>::value_type) == 0
+                )
             >
         >
         view<ValueT>