MDEV-15837: Assertion `item1->type() == Item::FIELD_ITEM && item2->type() == Item::FIELD_ITEM'

            failed in compare_order_elements function

The issue here is the function compare_order_lists() is called for the order by list of the window functions
so that those window function that can be computed together are adjacent.
So in the function compare_order_list we iterate over all the elements in the order list of the two functions and
compare the items in their order by clause.
The function compare_order_elements() is called for each item in the
order by clause. This function assumes that all the items that are in the order by list would be of the type
Item::FIELD_ITEM.

The case we have is that we have constants in the order by clause. We should ignore the constant and only compare
items of the type Item::FIELD_ITEM in compare_order_elements()

mysql-test/r/win.result

@@ -3506,5 +3506,17 @@ id	a	b	sum_a_b
 2	2	2	4
 drop table t1;
 #
+# MDEV-15837: Assertion `item1->type() == Item::FIELD_ITEM && item2->type() == Item::FIELD_ITEM'
+# failed in compare_order_elements function
+#
+CREATE TABLE t1 (a1 int);
+insert into t1 values (1),(2),(3);
+SELECT  rank() OVER (ORDER BY 1), ROW_NUMBER() OVER (ORDER BY (EXPORT_SET(5,'Y','N',',',4))) FROM t1;
+rank() OVER (ORDER BY 1)	ROW_NUMBER() OVER (ORDER BY (EXPORT_SET(5,'Y','N',',',4)))
+1	1
+1	2
+1	3
+drop table t1;
+#
 # End of 10.2 tests
 #

mysql-test/t/win.test

@@ -2255,6 +2255,16 @@ select e.id,
 from t1 e;
 drop table t1;
 
+--echo #
+--echo # MDEV-15837: Assertion `item1->type() == Item::FIELD_ITEM && item2->type() == Item::FIELD_ITEM'
+--echo # failed in compare_order_elements function
+--echo #
+
+CREATE TABLE t1 (a1 int);
+insert into t1 values (1),(2),(3);
+SELECT  rank() OVER (ORDER BY 1), ROW_NUMBER() OVER (ORDER BY (EXPORT_SET(5,'Y','N',',',4))) FROM t1;
+drop table t1;
+
 --echo #
 --echo # End of 10.2 tests
 --echo #

sql/sql_window.cc

@@ -342,6 +342,22 @@ int compare_order_lists(SQL_I_List<ORDER> *part_list1,
   for ( ; elem1 && elem2; elem1= elem1->next, elem2= elem2->next)
   {
     int cmp;
+    // remove all constants as we don't need them for comparision
+    while(elem1 && ((*elem1->item)->real_item())->const_item())
+    {
+      elem1= elem1->next;
+      continue;
+    }
+
+    while(elem2 && ((*elem2->item)->real_item())->const_item())
+    {
+      elem2= elem2->next;
+      continue;
+    }
+
+    if (!elem1 || !elem2)
+      break;
+
     if ((cmp= compare_order_elements(elem1, elem2)))
       return cmp;
   }