[#MDEV-29821] DATE/DATETIME null-safe equals should consider expressions that evaluate to NULL <=> NULL as equal

This was found on MariaDB 10.6.3. There are some inconsistencies in the below expressions using <=>, which I'd expect to all equal 1.

mariadb10.6.3> SELECT CAST(-1 AS DATETIME),

                      CAST(NULL AS DATETIME),

                      CAST(-1 AS DATE), CAST(NULL AS DATE),

                      CAST(-1 AS DATETIME) <=> CAST(NULL AS DATETIME),

                      CAST(-1 AS DATETIME) <=> NULL,

                      CAST(NULL AS DATETIME) <=> NULL,

                      CAST(-1 AS DATE) <=> CAST(NULL AS DATE),

                      CAST(-1 AS DATE) <=> NULL,

                      CAST(NULL AS DATE) <=> NULL, NULL <=> NULL\G

*************************** 1. row ***************************

                           CAST(-1 AS DATETIME): NULL

                         CAST(NULL AS DATETIME): NULL

                               CAST(-1 AS DATE): NULL

                             CAST(NULL AS DATE): NULL

CAST(-1 AS DATETIME) <=> CAST(NULL AS DATETIME): 0

                  CAST(-1 AS DATETIME) <=> NULL: 0

                CAST(NULL AS DATETIME) <=> NULL: 1

        CAST(-1 AS DATE) <=> CAST(NULL AS DATE): 0

                      CAST(-1 AS DATE) <=> NULL: 0

                    CAST(NULL AS DATE) <=> NULL: 1

                                  NULL <=> NULL: 1

1 row in set, 6 warnings (0.00 sec)

[MDEV-29821] DATE/DATETIME null-safe equals should consider expressions that evaluate to NULL <=> NULL as equal Created: 2022-10-19 Updated: 2023-11-28
Status:	Open
Project:	MariaDB Server
Component/s:	Data types
Affects Version/s:	10.6.3, 10.3.36, 10.5.17, 10.9.2
Fix Version/s:	10.4, 10.5, 10.6