CREATE TABLE t1 (a TIMESTAMP(6), b TIMESTAMP AS (a) VIRTUAL);

ALTER TABLE t1 ADD INDEX(b);

MariaDB [test]> show warnings;

+---------+------+--------------------------------------------------------------------------------------+

| Level   | Code | Message                                                                              |

+---------+------+--------------------------------------------------------------------------------------+

| Warning | 1901 | Function or expression '`a`' cannot be used in the GENERATED ALWAYS AS clause of `b` |

| Warning | 1105 | Expression depends on the @@sql_mode value TIME_ROUND_FRACTIONAL                     |

| Warning | 1901 | Function or expression '`a`' cannot be used in the GENERATED ALWAYS AS clause of `b` |

| Warning | 1105 | Expression depends on the @@sql_mode value TIME_ROUND_FRACTIONAL                     |

+---------+------+--------------------------------------------------------------------------------------+

4 rows in set (0.000 sec)