NFC: pointer_offset* functions always return void*

This requires that the caller perform the cast on the output rather than the
input, which is a little closer to the truth.  Shuffle some casts into the right
position.

src/ds/address.h

@@ -18,19 +18,19 @@ namespace snmalloc
   /**
    * Perform pointer arithmetic and return the adjusted pointer.
    */
-  template<typename T>
-  inline T* pointer_offset(T* base, size_t diff)
+  template<typename U = void, typename T>
+  inline U* pointer_offset(T* base, size_t diff)
   {
-    return reinterpret_cast<T*>(reinterpret_cast<char*>(base) + diff);
+    return reinterpret_cast<U*>(reinterpret_cast<char*>(base) + diff);
   }
 
   /**
    * Perform pointer arithmetic and return the adjusted pointer.
    */
-  template<typename T>
-  inline T* pointer_offset_signed(T* base, ptrdiff_t diff)
+  template<typename U = void, typename T>
+  inline U* pointer_offset_signed(T* base, ptrdiff_t diff)
   {
-    return reinterpret_cast<T*>(reinterpret_cast<char*>(base) + diff);
+    return reinterpret_cast<U*>(reinterpret_cast<char*>(base) + diff);
   }
 
   /**

src/ds/cdllist.h

@@ -31,7 +31,7 @@ namespace snmalloc
 
     SNMALLOC_FAST_PATH T* get_next()
     {
-      return pointer_offset_signed(static_cast<T*>(this), to_next);
+      return pointer_offset_signed<T>(this, to_next);
     }
   };