BUG #19709: Incorrect result when comparing OLD tuple column to itself in INSERT RETURNING

The following bug has been logged on the website:

Bug reference: 19709
Logged by: N J
Email address: 1482694023(at)qq(dot)com
PostgreSQL version: 18.4
Operating system: Windows 11 64-bit
Description:

Environment:
PostgreSQL: 18.4
OS: Windows 11 64-bit

Reproduction query:

CREATE TEMP TABLE IF NOT EXISTS a0 (x INTEGER);
TRUNCATE TABLE a0;
INSERT INTO a0 VALUES (42)
RETURNING
CASE
WHEN (old).x IS DISTINCT FROM old.x THEN 1
ELSE 0
END AS bug;

Observed result:
bug
-----
1

Expected result:
bug
-----
0

Explanation:
For INSERT statements, the OLD tuple is all NULL.
The expression old.x IS DISTINCT FROM old.x compares NULL with NULL.
Per SQL standard, NULL IS DISTINCT FROM NULL returns false, so the CASE
expression should return 0.
PostgreSQL incorrectly evaluates this predicate to true and returns 1.