Rounding functions create a wrong data type for integer input

CREATE OR REPLACE TABLE t1 (a BIGINT);

CREATE OR REPLACE TABLE t2 AS SELECT

  a, ROUND(a), TRUNCATE(a,0), FLOOR(a), CEILING(a) FROM t1;

DESC t2;

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

| Field         | Type          | Null | Key | Default | Extra |

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

| a             | bigint(20)    | YES  |     | NULL    |       |

| ROUND(a)      | bigint(19)    | YES  |     | NULL    |       |

| TRUNCATE(a,0) | bigint(19)    | YES  |     | NULL    |       |

| FLOOR(a)      | decimal(19,0) | YES  |     | NULL    |       |

| CEILING(a)    | decimal(19,0) | YES  |     | NULL    |       |

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