Non-strict XMLTYPE cast returns NULL but IS NULL evaluates false

In MariaDB 12.3.3 on Linux/amd64, casting invalid XML text to XMLTYPE in a non-strict session returns NULL, but applying IS NULL to the same expression returns false. A WHERE clause using the predicate consequently omits the affected row.

Reproducer

CREATE DATABASE xml_cast_case;

USE xml_cast_case;

CREATE TABLE xml_samples(id INT PRIMARY KEY,a VARCHAR(20) NOT NULL) ENGINE=InnoDB;

INSERT INTO xml_samples VALUES(1,'<a/>'),(2,'text');

SET SESSION sql_mode='';

SELECT 'c_projection',id,CAST(a AS XMLTYPE) FROM xml_samples ORDER BY id;

SELECT 'c_predicate',id,CAST(a AS XMLTYPE) IS NULL FROM xml_samples ORDER BY id;

SELECT 'c_filter',COUNT(*) FROM xml_samples WHERE CAST(a AS XMLTYPE) IS NULL;

SET SESSION sql_mode='STRICT_TRANS_TABLES';

SELECT 'c_strict',id,CAST(a AS XMLTYPE) IS NULL FROM xml_samples ORDER BY id;

SELECT 'c_strict_filter',COUNT(*) FROM xml_samples WHERE CAST(a AS XMLTYPE) IS NULL;

The client returns:

c_projection	1	<a/>

c_projection	2	NULL

c_predicate	1	0

c_predicate	2	0

c_filter	0

c_strict	1	0

c_strict	2	1

c_strict_filter	1

The non-strict projection returns NULL for row 2, while IS NULL returns 0 and the filter matches no rows. In strict mode, IS NULL returns 1 for the same row and the filter matches it.

Whenever CAST(a AS XMLTYPE) evaluates to NULL, CAST(a AS XMLTYPE) IS NULL should evaluate to true, and a WHERE clause using that predicate should select the row. This follows from SQL null-predicate semantics and should not depend on strictness mode.

Enabling strict mode or validating XML before casting provides a workaround.

This occurs when invalid XML text is cast to XMLTYPE and tested with IS NULL in a non-strict session.