ALTER TABLE DROP CONSTRAINT does not work for unique constraints

The syntax is supported in MariaDB 10.2.1 (https://mariadb.com/kb/en/library/alter-table/#drop-constraint), but it doesn't work for unique constraints:

create table t (

  a int,

  b int,

  constraint x unique (a),

  constraint y check (b = 1)

);

-- Does not work:

alter table t drop constraint x;

-- These work:

alter table t drop key x;

alter table t drop constraint y;

Note, this looks like a duplicate of MDEV-10038, but at the time of MDEV-10038, the syntax wasn't yet supported by MariaDB. Now it is. Feel free to close this as a duplicate of the other (and reopen the other), if you think that's the better approach.