Re: jsonb unique constraints

Can you have unique constraints on jsonb columns keys? I've looked for examples but haven't found any what is the proper syntax? Here's what I tried:

CREATE TABLE report_json (
    recnum int,
    id integer,
    report jsonb,
    PRIMARY KEY (recnum),
    CONSTRAINT report_json_unique_constraint UNIQUE (id,((report ->> 'data')::int),((report ->> 'ctc')::int),((report ->> 'dtg')::int)
);

which causes a syntax error at the first '(' around 'report ->>'.