Bug #19612819 : FILESORT: ASSERTION FAILED: POS->FIELD != 0 || POS->ITEM != 0

Problem:
While getting the temp table field for a REF_ITEM
make_sortorder is using the real_item. As a result
server fails later with an assert.

Solution:
Do not use real_item to get the temp table field.
Instead use the REF_ITEM itself as temp table fields
are created for REF_ITEM not the real_item.

sql/sql_select.cc

-/* Copyright (c) 2000, 2014, Oracle and/or its affiliates. All rights reserved.
+/* Copyright (c) 2000, 2015, Oracle and/or its affiliates. All rights reserved.
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -14649,18 +14649,33 @@ SORT_FIELD *make_unireg_sortorder(ORDER *order, uint *length,
 
   for (;order;order=order->next,pos++)
   {
-    Item *item= order->item[0]->real_item();
+    Item *const item= order->item[0], *const real_item= item->real_item();
     pos->field= 0; pos->item= 0;
-    if (item->type() == Item::FIELD_ITEM)
-      pos->field= ((Item_field*) item)->field;
-    else if (item->type() == Item::SUM_FUNC_ITEM && !item->const_item())
-      pos->field= ((Item_sum*) item)->get_tmp_table_field();
-    else if (item->type() == Item::COPY_STR_ITEM)
-    {						// Blob patch
-      pos->item= ((Item_copy*) item)->get_item();
+    if (real_item->type() == Item::FIELD_ITEM)
+    {
+      // Could be a field, or Item_direct_view_ref wrapping a field
+      DBUG_ASSERT(item->type() == Item::FIELD_ITEM ||
+                  (item->type() == Item::REF_ITEM &&
+                   static_cast<Item_ref*>(item)->ref_type() ==
+                   Item_ref::VIEW_REF));
+      pos->field= static_cast<Item_field*>(real_item)->field;
+    }
+    else if (real_item->type() == Item::SUM_FUNC_ITEM &&
+             !real_item->const_item())
+    {
+      // Aggregate, or Item_aggregate_ref
+      DBUG_ASSERT(item->type() == Item::SUM_FUNC_ITEM ||
+                  (item->type() == Item::REF_ITEM &&
+                   static_cast<Item_ref*>(item)->ref_type() ==
+                   Item_ref::AGGREGATE_REF));
+      pos->field= item->get_tmp_table_field();
+    }
+    else if (real_item->type() == Item::COPY_STR_ITEM)
+    {                                           // Blob patch
+      pos->item= static_cast<Item_copy*>(real_item)->get_item();
     }
     else
-      pos->item= *order->item;
+      pos->item= item;
     pos->reverse=! order->asc;
   }
   *length=count;