MDEV-28094 Window function in expression in ORDER BY

call item->split_sum_func() in setup_order() just as
it's done in setup_fields()

mysql-test/main/win_sum.result

@@ -93,3 +93,15 @@ pk	a	c	sum(c) over (partition by a order by pk ROWS BETWEEN 1 PRECEDING AND 1 FO
 126	6	NULL	NULL
 127	6	NULL	NULL
 drop table t1;
+#
+# End of 10.2 tests
+#
+#
+# MDEV-28094 Window function in expression in ORDER BY
+#
+SELECT EXISTS (SELECT 1 ORDER BY 1+sum(2) OVER ());
+EXISTS (SELECT 1 ORDER BY 1+sum(2) OVER ())
+1
+#
+# End of 10.4 tests
+#

mysql-test/main/win_sum.test

@@ -45,3 +45,16 @@ select pk, a, c, sum(c) over (partition by a order by pk ROWS BETWEEN 1 PRECEDIN
 from t1;
 
 drop table t1;
+
+--echo #
+--echo # End of 10.2 tests
+--echo #
+
+--echo #
+--echo # MDEV-28094 Window function in expression in ORDER BY
+--echo #
+SELECT EXISTS (SELECT 1 ORDER BY 1+sum(2) OVER ());
+
+--echo #
+--echo # End of 10.4 tests
+--echo #

sql/sql_select.cc

@@ -24546,8 +24546,8 @@ int setup_order(THD *thd, Ref_ptr_array ref_pointer_array, TABLE_LIST *tables,
       return 1;
     }
 
-    if (from_window_spec && item->with_sum_func() &&
-        item->type() != Item::SUM_FUNC_ITEM)
+    if ((from_window_spec && item->with_sum_func() &&
+         item->type() != Item::SUM_FUNC_ITEM) || item->with_window_func)
     {
       item->split_sum_func(thd, ref_pointer_array,
                            all_fields, SPLIT_SUM_SELECT);