BUG #15128: Erroneous inner query is executing with wrong results

The following bug has been logged on the website:

Bug reference: 15128
Logged by: Yossi Eilaty
Email address: yossi(dot)eilaty(at)gong(dot)io
PostgreSQL version: 9.6.2
Operating system: x86_64-pc-linux-gnu, compiled by gcc (Debian 4.9.2
Description:

Consider the following tables:
CREATE TABLE a (
r BIGINT,
y BIGINT
);
CREATE TABLE b (
x BIGINT,
y BIGINT
);

with the following data:

INSERT INTO a (r, y) VALUES (1, 1);
INSERT INTO a (r, y) VALUES (2, 2);

INSERT INTO b (x, y) VALUES (1, 1);
INSERT INTO b (x, y) VALUES (2, 2);

If I run the following query:

SELECT y
FROM b
WHERE x IN (SELECT x
FROM (SELECT r
FROM a
WHERE y = 1) the_x);

I get, as a result, the y value of all rows of table b, which is wrong since
the inner query

SELECT x
FROM (SELECT r
FROM a
WHERE y = 1) the_x

is wrong and if run solely returns the error "[42703] ERROR: column "x" does
not exist", which is correct.

The same thing happens for the following query:

SELECT y
FROM b
WHERE x IN (SELECT x
FROM (SELECT x
FROM a
WHERE y = 1) the_x);

where, again, the innermost query is wrong (there's no x in a).