ROW_NUMBER is not set and differs from the message upon WARN_DATA_TRUNCATED produced by inplace ALTER

--source include/have_innodb.inc

create or replace table t (a varchar(64)) engine=InnoDB;

insert into t values ('foo'),(null);

--error WARN_DATA_TRUNCATED

alter table t add primary key(a), algorithm=INPLACE;

get diagnostics condition 1 @n= row_number, @m= message_text;

select @n, @m;

MariaDB [test]> get diagnostics condition 1 @n= row_number, @m= message_text; select @n, @m;

Query OK, 0 rows affected (0.000 sec)

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

| @n   | @m                                     |

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

|    0 | Data truncated for column 'a' at row 2 |

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

1 row in set (0.000 sec)