BUG #14969: Delta in 9.6 and 10

The following bug has been logged on the website:

Bug reference: 14969
Logged by: Mohan Patil
Email address: mohanpatil048(at)gmail(dot)com
PostgreSQL version: 10.1
Operating system: ubuntu 14.04
Description:

When I execute the below query in 9.6 version,I get the result.

But when I execute it on 10.1, it is giving the error,

********** Error **********

ERROR: set-returning functions are not allowed in CASE
SQL state: 0A000
Hint: You might be able to move the set-returning function into a LATERAL
FROM item.

QUERY::
SELECT t.id, t.name
FROM (SELECT ( CASE
WHEN CARDINALITY(tier_ids) > 1 THEN UNNEST(tier_ids)
ELSE NULL
END ) AS tier_id
FROM relation
WHERE id = 1001) AS d
INNER JOIN tier t ON t.id = d.tier_id ;