ALTER USER IF EXISTS does not work, although documentation says it should

ALTER USER [IF EXISTS] 

 user_specification [,user_specification] ...

  [REQUIRE {NONE | tls_option [[AND] tls_option] ...}]

  [WITH resource_option [resource_option] ...]

MariaDB [test]> CREATE USER foo@bar;

Query OK, 0 rows affected (0.09 sec)

MariaDB [test]> ALTER USER IF EXISTS foo@bar REQUIRE SSL;

ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'IF EXISTS foo@bar REQUIRE SSL' at line 1