MDEV-28095 crash in multi-update and implicit grouping

disallow implicit grouping in multi-update.
explicit GROUP BY is not allowed by the grammar.

mysql-test/main/multi_update_innodb.result

@@ -207,4 +207,19 @@ ERROR 23000: Duplicate entry '0000-00-00 00:00:00' for key 'f2k'
 DROP VIEW v1;
 DROP TABLE t3,t4;
 SET @@sql_mode=@save_sql_mode;
+#
 # End of 10.2 tests
+#
+#
+# MDEV-28095 crash in multi-update and implicit grouping
+#
+CREATE TABLE t1 (a int) engine=innodb;
+INSERT INTO t1 VALUES (1),(2);
+CREATE TABLE t2 (b int);
+INSERT INTO t2 VALUES (1),(2);
+UPDATE t1 NATURAL JOIN t2 SET a = 1 ORDER BY AVG (a) ;
+ERROR HY000: Invalid use of group function
+DROP TABLE t1, t2;
+#
+# End of 10.3 tests
+#

mysql-test/main/multi_update_innodb.test

@@ -243,4 +243,23 @@ DROP VIEW v1;
 DROP TABLE t3,t4;
 SET @@sql_mode=@save_sql_mode;
 
+--echo #
 --echo # End of 10.2 tests
+--echo #
+
+--echo #
+--echo # MDEV-28095 crash in multi-update and implicit grouping
+--echo #
+CREATE TABLE t1 (a int) engine=innodb;
+INSERT INTO t1 VALUES (1),(2);
+CREATE TABLE t2 (b int);
+INSERT INTO t2 VALUES (1),(2);
+--error ER_INVALID_GROUP_FUNC_USE
+UPDATE t1 NATURAL JOIN t2 SET a = 1 ORDER BY AVG (a) ;
+DROP TABLE t1, t2;
+
+
+--echo #
+--echo # End of 10.3 tests
+--echo #
+

sql/sql_update.cc

@@ -2150,6 +2150,11 @@ multi_update::initialize_tables(JOIN *join)
   if (unlikely((thd->variables.option_bits & OPTION_SAFE_UPDATES) &&
                error_if_full_join(join)))
     DBUG_RETURN(1);
+  if (join->implicit_grouping)
+  {
+    my_error(ER_INVALID_GROUP_FUNC_USE, MYF(0));
+    DBUG_RETURN(1);
+  }
   main_table=join->join_tab->table;
   table_to_update= 0;