BUG #19699: LIKE with a trailing escape fails to raise SQLSTATE 22025 for empty input

The following bug has been logged on the website:

Bug reference: 19699
Logged by: Qifan Liu
Email address: imchifan(at)163(dot)com
PostgreSQL version: 18.6
Operating system: Linux/amd64
Description:

PostgreSQL version: PostgreSQL 20devel at
a12600b762c36d91450ce085fa25ef75250bc1c2; PostgreSQL 18.6; PostgreSQL 17.11
Operating system: Linux/amd64

Description
-----------
LIKE does not reject a pattern ending in its active escape character when
the input string is empty. Both the default backslash escape and a custom
escape silently return instead of raising SQLSTATE 22025. The equivalent
cases with nonempty input raise 22025.

Steps to reproduce
------------------
Run the following input with psql:

BEGIN;

CREATE TEMP TABLE bugseer_postgres_00013_like_escape_results
(
case_name text PRIMARY KEY,
returned_sqlstate text
);

DO $block$
DECLARE
state text;
BEGIN
PERFORM ''::text LIKE E'\\';
INSERT INTO bugseer_postgres_00013_like_escape_results VALUES
('empty_default', NULL);
EXCEPTION WHEN OTHERS THEN
GET STACKED DIAGNOSTICS state = RETURNED_SQLSTATE;
INSERT INTO bugseer_postgres_00013_like_escape_results VALUES
('empty_default', state);
END
$block$;

DO $block$
DECLARE
state text;
BEGIN
PERFORM 'x'::text LIKE E'\\';
INSERT INTO bugseer_postgres_00013_like_escape_results VALUES
('nonempty_default', NULL);
EXCEPTION WHEN OTHERS THEN
GET STACKED DIAGNOSTICS state = RETURNED_SQLSTATE;
INSERT INTO bugseer_postgres_00013_like_escape_results VALUES
('nonempty_default', state);
END
$block$;

DO $block$
DECLARE
state text;
BEGIN
PERFORM ''::text LIKE '#' ESCAPE '#';
INSERT INTO bugseer_postgres_00013_like_escape_results VALUES
('empty_custom', NULL);
EXCEPTION WHEN OTHERS THEN
GET STACKED DIAGNOSTICS state = RETURNED_SQLSTATE;
INSERT INTO bugseer_postgres_00013_like_escape_results VALUES
('empty_custom', state);
END
$block$;

DO $block$
DECLARE
state text;
BEGIN
PERFORM 'x'::text LIKE '#' ESCAPE '#';
INSERT INTO bugseer_postgres_00013_like_escape_results VALUES
('nonempty_custom', NULL);
EXCEPTION WHEN OTHERS THEN
GET STACKED DIAGNOSTICS state = RETURNED_SQLSTATE;
INSERT INTO bugseer_postgres_00013_like_escape_results VALUES
('nonempty_custom', state);
END
$block$;

TABLE bugseer_postgres_00013_like_escape_results;

SELECT count(*) = 4 AS all_cases_ran,
bool_and(coalesce(returned_sqlstate = '22025', false)) AS
oracle_all_rejected
FROM bugseer_postgres_00013_like_escape_results;

ROLLBACK;

Actual result
-------------
case_name | returned_sqlstate
------------------+-------------------
empty_default |
nonempty_default | 22025
empty_custom |
nonempty_custom | 22025
(4 rows)

all_cases_ran | oracle_all_rejected
---------------+---------------------
t | f
(1 row)

Expected result
---------------
Every pattern ending in its active escape character should raise SQLSTATE
22025, regardless of whether the input string is empty or nonempty. All four
returned_sqlstate values should therefore be 22025 and oracle_all_rejected
should be true.

Additional information
----------------------
The issue was reproduced on PostgreSQL 20devel, PostgreSQL 18.6, and
PostgreSQL 17.11.