BUG #13988: "plan should not reference subplan's variable" whilst using row level security

The following bug has been logged on the website:

Bug reference:      13988
Logged by:          Adam Guthrie
Email address:      asguthrie@gmail.com
PostgreSQL version: 9.5.1
Operating system:   OS X 10.11
Description:

The following sql:

****

CREATE TABLE a (
    id      INTEGER PRIMARY KEY
);

CREATE TABLE b (
    id      INTEGER PRIMARY KEY,
    a_id    INTEGER,
    text    TEXT
);

CREATE POLICY a_select ON b FOR SELECT
    USING ( EXISTS(SELECT FROM a WHERE a.id = b.a_id) );
ALTER TABLE b ENABLE ROW LEVEL SECURITY;

INSERT INTO a (id) VALUES (1);

INSERT INTO b (id, a_id, text) VALUES (1, 1, 'one');

CREATE ROLE test;
GRANT ALL ON ALL TABLES IN SCHEMA public TO test;
SET ROLE test;

SELECT * FROM b;

UPDATE b SET text = 'ONE' WHERE id = 1;

****

gives:

psql:/tmp/plan.sql:25: ERROR:  XX000: plan should not reference subplan's
variable
LOCATION:  finalize_plan, subselect.c:2624

whereas I believe it should give:

UPDATE 0