Virtual column type cannot be converted from one to another - unhelpful error message

MariaDB [test]> show create table  bad_with_key\G

*************************** 1. row ***************************

       Table: bad_with_key

Create Table: CREATE TABLE `bad_with_key` (

  `id` varchar(10) CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci NOT NULL,

  `id_md5` char(32) GENERATED ALWAYS AS (md5(`id`)) VIRTUAL,

  `f` float DEFAULT NULL,

  `ts` timestamp NOT NULL DEFAULT current_timestamp() ON UPDATE current_timestamp(),

  UNIQUE KEY `id` (`id`,`id_md5`)

) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4

1 row in set (0.000 sec)

MariaDB [test]> alter table bad_with_key change column id_len id_md5 binary(16) generated always as (unhex(md5(id))) PERSISTENT AFTER id;

ERROR 1054 (42S22): Unknown column 'id_len' in 'bad_with_key'

MariaDB [test]> alter table bad_with_key modify  column id_md5 binary(16) generated always as (unhex(md5(id))) PERSISTENT AFTER id;

ERROR 1907 (HY000): This is not yet supported for generated columns

MariaDB [test]> alter table bad_with_key modify column id_md5 char(32) generated always as (unhex(md5(id))) PERSISTENT AFTER id;

ERROR 1907 (HY000): This is not yet supported for generated columns

MariaDB [test]> alter table bad_with_key modify column id_md5 char(32) generated always as (md5(id)) PERSISTENT AFTER id;

ERROR 1907 (HY000): This is not yet supported for generated columns

MariaDB [test]> alter table bad_with_key modify  column id_md5 binary(16) as (unhex(md5(id))) PERSISTENT AFTER id;

ERROR 1907 (HY000): This is not yet supported for generated columns

MariaDB [test]> alter table bad_with_key modify  column id_md5 char(32) as (unhex(md5(id))) PERSISTENT AFTER id;

ERROR 1907 (HY000): This is not yet supported for generated columns

MariaDB [test]> alter table bad_with_key drop index id;

Query OK, 0 rows affected (0.030 sec)

Records: 0  Duplicates: 0  Warnings: 0

MariaDB [test]> alter table bad_with_key modify  column id_md5 binary(16) generated always as (unhex(md5(id))) PERSISTENT AFTER id;

ERROR 1907 (HY000): This is not yet supported for generated columns

MariaDB [test]> show create table bad_with_key

    -> \G

*************************** 1. row ***************************

       Table: bad_with_key

Create Table: CREATE TABLE `bad_with_key` (

  `id` varchar(10) CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci NOT NULL,

  `id_md5` char(32) GENERATED ALWAYS AS (md5(`id`)) VIRTUAL,

  `f` float DEFAULT NULL,

  `ts` timestamp NOT NULL DEFAULT current_timestamp() ON UPDATE current_timestamp()

) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4

1 row in set (0.000 sec)

MariaDB [test]> alter table bad_with_key drop column  id_md5;

Query OK, 0 rows affected (0.035 sec)

Records: 0  Duplicates: 0  Warnings: 0

MariaDB [test]> alter table bad_with_key add column id_md5 binary(16) generated always as (unhex(md5(id))) PERSISTENT AFTER id;

Query OK, 8 rows affected (0.059 sec)

Records: 8  Duplicates: 0  Warnings: 0