Indexed generated-column substitution returns no row for an equivalent VARCHAR predicate

MariaDB 12.3.3 returns no row when it substitutes a VARCHAR expression with an indexed generated column that is shorter than the expression result. The same predicate returns row 1 when the index substitution is disabled.

Reproducer

Run the following SQL in one session:

CREATE DATABASE vcol_test;

USE vcol_test;

SET SESSION sql_mode='';

CREATE TABLE t1 (

  id INT PRIMARY KEY,

  a VARCHAR(10) CHARACTER SET latin1 COLLATE latin1_bin,

  v VARCHAR(3) CHARACTER SET latin1 COLLATE latin1_bin

    AS (CONCAT(a,'')),

  KEY v_idx(v)

) ENGINE=InnoDB;

INSERT INTO t1(id,a) VALUES (1,'abcd'),(2,'abc'),(3,'xyz');

SHOW WARNINGS;

SELECT id,a,v FROM t1 ORDER BY id;

SELECT 'baseline' AS check_name,

       COALESCE(GROUP_CONCAT(id ORDER BY id),'EMPTY') AS ids

FROM t1 IGNORE INDEX(v_idx)

WHERE CONCAT(a,'')='abcd';

SELECT 'optimized' AS check_name,

       COALESCE(GROUP_CONCAT(id ORDER BY id),'EMPTY') AS ids

FROM t1

WHERE CONCAT(a,'')='abcd';

EXPLAIN EXTENDED

SELECT id FROM t1 WHERE CONCAT(a,'')='abcd';

SHOW WARNINGS;

DROP DATABASE vcol_test;

The empty SQL mode permits the generated value to be truncated and produces the expected warning. It does not alter the source value in a.

Recorded result

Level    Code  Message

Warning  1265  Data truncated for column 'v' at row 1

id  a     v

1   abcd  abc

2   abc   abc

3   xyz   xyz

check_name  ids

baseline    1

check_name  ids

optimized   EMPTY

EXPLAIN EXTENDED used v_idx, and SHOW WARNINGS showed that the predicate had been rewritten from CONCAT(a,'')='abcd' to v='abcd'.

Expected behavior

Both queries have the same predicate and should return row 1. The stored source value is still abcd, so using the narrower generated value abc for the comparison must not change the result set. Either the substitution should preserve the predicate's semantics or it should not be applied when the generated column is narrower.

This occurs when a WHERE expression is substituted with an indexed generated column whose VARCHAR length is narrower than the source expression.