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(at)gmail(dot)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