BUG #14704: How to create unique index with a case statement?

The following bug has been logged on the website:

Bug reference: 14704
Logged by: Naveen Vemula
Email address: naveen7eceindia(at)gmail(dot)com
PostgreSQL version: 9.6.0
Operating system: Windows
Description:

CREATE UNIQUE INDEX XAK2TRADPAADR_TD ON TRADPAADR_TD(TPID,
case TDADRTYP when 'BY' then 0
else TDID
end,
case TDADRNO when '1' then 0
else TDID
end,
case TDLANG when 'ENG' then 0
else 1
end);

ERROR: syntax error at or near "case"
LINE 4: case TDADRTYP
^
********** Error **********

ERROR: syntax error at or near "case"
SQL state: 42601
Character: 72

Here TDADRTYP - varchar(4)
TDID - bigint
TDADRNO - varchar(15)
TDLANG - varchar(4)