Pluggable data type fails type aggregation with NULL and string operands

INSTALL SONAME 'type_test';

CREATE TABLE t (g INT, i test_int8, d test_double);

INSERT INTO t VALUES (1, 10, 1.5), (5, 20, 2.5);

SELECT i + NULL FROM t;

-- ERROR 4078 (HY000): Illegal parameter data types test_int8 and null for operation '+'

SELECT i + '3' FROM t;                     -- 4078, operation '+'

SELECT CASE WHEN g>3 THEN i ELSE NULL END FROM t;   -- 4078, 'case'

SELECT CASE WHEN g>3 THEN i ELSE 'x'  END FROM t;   -- 4078, 'case'

SELECT COALESCE(i, NULL) FROM t;           -- 4078, 'coalesce'

SELECT IFNULL(d, NULL)   FROM t;           -- 4078, 'ifnull'

SELECT LEAST(i, NULL)    FROM t;           -- 4078, 'least'

SELECT i FROM t UNION SELECT NULL;         -- 4078

SELECT i FROM t UNION SELECT 'x';          -- 4078

13.1.0-opt>INSTALL SONAME 'type_test';

Query OK, 0 rows affected (0.002 sec)

13.1.0-opt>CREATE TABLE t (g INT, i test_int8, d test_double);

Query OK, 0 rows affected (0.008 sec)

13.1.0-opt>INSERT INTO t VALUES (1, 10, 1.5), (5, 20, 2.5);

Query OK, 2 rows affected (0.001 sec)

Records: 2  Duplicates: 0  Warnings: 0

13.1.0-opt>

13.1.0-opt>SELECT i + NULL FROM t;

ERROR 4078 (HY000): Illegal parameter data types test_int8 and null for operation '+'

13.1.0-opt>

13.1.0-opt>SELECT i + '3' FROM t;

ERROR 4078 (HY000): Illegal parameter data types test_int8 and varchar for operation '+'

13.1.0-opt>