A BOOLEAN != NOT NOT BOOLEAN predicate evaluates true in projection but false in filtering

`BOOLEAN` is `TINYINT(1)`, and inserting `127` preserves that non-canonical non-zero value. Therefore `NOT (NOT t1.c0)` is `1`, so `t1.c0 != (NOT (NOT t1.c0))` is `127 != 1`, i.e. true. MariaDB's projection evaluation reports that value as true, so the `WHERE` clause that builds `join_temp_0` should keep the row. Instead, MariaDB filters it away, and the final query incorrectly becomes empty.

How to repeat:

CREATE TABLE t0(c0 INT);
CREATE TABLE t1(c0 BOOLEAN PRIMARY KEY);

INSERT INTO t0 VALUES (0);
INSERT INTO t1 VALUES (127);

CREATE TEMPORARY TABLE join_temp_0 (c0 TINYINT(1) NOT NULL);
INSERT INTO join_temp_0 SELECT * FROM t1 WHERE (t1.c0 != (NOT (NOT t1.c0)));

SELECT MIN(t1.c0)
FROM t0 INNER JOIN join_temp_0 AS t1
ON ((t0.c0 < t1.c0) AND (t1.c0 != true))
HAVING (MIN(t1.c0) != true); – Expected correct result: 127 – actual Wrong result: <empty>