Materializing an empty `TIME` branch changes a `DATE` value in a mixed-type `UNION`

MariaDB returns different values and result metadata for two relationally
equivalent `UNION` queries. The query has three branches:

1. An empty branch whose expression type is `TIME`.
2. A branch returning one `DATE` value.
3. An empty integer branch.

When all expressions appear directly in the `UNION`, MariaDB returns the date as
`2026-05-21`. When only the empty `TIME` branch is put behind a CTE, derived
table, or temporary-table boundary, MariaDB returns
`2026-05-21 00:00:00` instead.

The empty branches return no rows in either query, but their expression types
still participate in `UNION` type negotiation. Moving an empty branch across a
relational boundary should not change the value produced by another branch.

1. How to repeat

No base tables or test data are required. Select an isolated database so that
the temporary-table variant can also be executed:

```sql

DROP DATABASE IF EXISTS date_union_type_test;

CREATE DATABASE date_union_type_test;

USE date_union_type_test;

```

Run the direct query:

```sql

(SELECT CAST('01:02:03' AS TIME) AS d WHERE FALSE)

UNION

(SELECT CAST('2026-05-21' AS DATE))

UNION ALL

SELECT 1 WHERE FALSE;

```

Result:

```text

2026-05-21

```

Now place only the empty `TIME` branch in a CTE:

```sql

WITH time_cut AS (

    SELECT CAST('01:02:03' AS TIME) AS d

    WHERE FALSE

)

(SELECT d FROM time_cut

 UNION

 SELECT CAST('2026-05-21' AS DATE))

UNION ALL

SELECT 1 WHERE FALSE;

```

Result:

```text

2026-05-21 00:00:00

```

The derived-table form produces the same changed result:

```sql

(SELECT d

 FROM (

     SELECT CAST('01:02:03' AS TIME) AS d

     WHERE FALSE

 ) AS time_cut

 UNION

 SELECT CAST('2026-05-21' AS DATE))

UNION ALL

SELECT 1 WHERE FALSE;

```

The temporary-table form also produces the changed result:

```sql

CREATE TEMPORARY TABLE time_cut AS

SELECT CAST('01:02:03' AS TIME) AS d

WHERE FALSE;

(SELECT d FROM time_cut

 UNION

 SELECT CAST('2026-05-21' AS DATE))

UNION ALL

SELECT 1 WHERE FALSE;

```

1. Metadata verification

With PyMySQL, the direct and rewritten queries both expose a string result, but
their declared maximum lengths differ:

```text

Direct query:       MYSQL_TYPE_VAR_STRING, length 40

CTE/derived query:  MYSQL_TYPE_VAR_STRING, length 76

```

The length change corresponds to MariaDB formatting the `DATE` branch as a
date-only string in the direct query and as a full `DATETIME` string after the
boundary.

1. Expected result

All equivalent forms should return the same value and compatible metadata. The
empty `TIME` branch contains no rows, and moving it into a CTE, derived table, or
temporary table does not change the relational result.

```text

2026-05-21

```

1. Actual result

The direct query returns:

```text

2026-05-21

```

The CTE, derived-table, and temporary-table forms return:

```text

2026-05-21 00:00:00

```