SIGABRT / assertion failure in st_select_lex_unit::optimize() at sql_union.cc:1124 when executing IN (SELECT ... INTERSECT SELECT ...)

Summary

MariaDB debug build crashes with SIGABRT in st_select_lex_unit::optimize() at
sql_union.cc:1124 while executing queries that use INTERSECT (and similarly EXCEPT) inside IN
/ EXISTS subqueries.

The assertion is:

DBUG_ASSERT(0);

This is reached when ha_enable_indexes(HA_KEY_SWITCH_ALL) returns non-zero during UNION
DISTINCT/INTERSECT subquery optimization.

Environment

• Version: 10.2.5-MariaDB-debug
• Platform: Linux
• Port: 3307
• Build: debug build from local source tree

Assertion

mysqld: /data/mariadb/sql/sql_union.cc:1124:
bool st_select_lex_unit::optimize(): Assertion `0' failed.

Source location:

if (union_distinct && table->file->ha_enable_indexes(HA_KEY_SWITCH_ALL))

{ DBUG_ASSERT(0); }

Minimal Reproducer

CREATE TABLE test.t_in_intersect_test (
id INT PRIMARY KEY AUTO_INCREMENT,
value INT
);

INSERT INTO test.t_in_intersect_test(value) VALUES (1),(2),(3),(4),(5);

SELECT /*+ IN_SUBQUERY_INTERSECT_NOMATCH */ id, value
FROM test.t_in_intersect_test
WHERE value IN (SELECT 1 INTERSECT SELECT 2);

Expected Result

The query should return an empty result set.

Actual Result

The server aborts with SIGABRT and crashes.