Create unique index oracle with where clause

I m trying to create a unique index with where clause like this:

CREATE UNIQUE INDEX cmt_unique_sid_yid_ct
          ON COMMENTARY (source_id, year_id)
       WHERE comment_type_id = (
         select comment_type_name 
         from comment_type 
         where comment_type_name = 'Final'
       );

Getting this error :

02158. 00000 -  "invalid CREATE INDEX option"
*Cause:    An option other than COMPRESS, NOCOMPRESS, PCTFREE, INITRANS,
           MAXTRANS, STORAGE, TABLESPACE, PARALLEL, NOPARALLEL, RECOVERABLE,
           UNRECOVERABLE, LOGGING, NOLOGGING, LOCAL, or GLOBAL was specified.
*Action:   Choose one of the valid CREATE INDEX options.

I m basically trying restrict comments with type Final from comment_type table, to only one row per source/year. So scope to :

comment_type_name | source_id | year_id

How can I do this with SQL, specifically oracle?

No such thing in Oracle, just some workaround with function based indexes:

CREATE UNIQUE INDEX cmt_unique_sid_yid_ct
ON COMMENTARY (
  case when comment_type_name = 'Final' then source_id end,
  case when comment_type_name = 'Final' then year_id end
);

Sample query that can use this index:

select * from commentary
where 
case when comment_type_name = 'Final' then source_id end = :B1
and case when comment_type_name = 'Final' then year_id end = :B2
;