TIMESTAMP '<date>' returns ER_WRONG_VALUE (Incorrect DATETIME value)

On some reason, the following returns an error:

MariaDB [test]> select timestamp '2021-12-12';

ERROR 1525 (HY000): Incorrect DATETIME value: '2021-12-12'

MariaDB [test]> select timestamp '20211212';

ERROR 1525 (HY000): Incorrect DATETIME value: '20211212'

while this, for example, works:

MariaDB [test]> select timestamp '2021-12-12 0';

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

| timestamp '2021-12-12 0' |

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

| 2021-12-12 00:00:00      |

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

1 row in set (0.000 sec)

This also works

MariaDB [test]> select cast('2021-12-12' as datetime);

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

| cast('2021-12-12' as datetime) |

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

| 2021-12-12 00:00:00            |

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

1 row in set (0.000 sec)

and naturally a date can be inserted into a timestamp/datetime column.

bar said the error upon timestamp literal may be a bug, so I'm filing it so that it's at least tracked.

I cannot find a definitive proof in documentation whether it should work or not. MySQL manual, while quite verbose, doesn't say explicitly whether it should work. It has, however, this note:

Values specified as numbers should be 6, 8, 12, or 14 digits long. If a number is 8 or 14 digits long, it is assumed to be in YYYYMMDD or YYYYMMDDhhmmss format and that the year is given by the first 4 digits

So, it appears that at least timestamp '20211212' should work (but it doesn't work in MySQL either).

MariaDB documentation is very confusing, it doesn't help at all.

One non-obvious effect is that in 10.7, --as-of option added to mysqldump in the scope of MDEV-16355 doesn't work with date-only values. It happens because the value is converted into a statement like

SELECT * FROM <table> FOR SYSTEM_TIME AS OF TIMESTAMP '<value>';