version 21.5.5.12 (official build)

I'm trying to change data type for a column that is used in the TTL expression.
It works well when the table is empty.
When the table is not empty, the mutation fails.
As a result the table ends up in an inconsistent state.

```
create table test
(
InsertionDateTime DateTime,
TTLDays Int32 DEFAULT CAST(365, 'Int32')
)
Engine=MergeTree()
order by tuple()
TTL InsertionDateTime + toIntervalDay(TTLDays);

Ok.

insert into test values (now(), 23);

Ok.

ALTER TABLE test modify column TTLDays Int16 DEFAULT CAST(365, 'Int16');

Code: 341, e.displayText() = DB::Exception: Exception happened during execution of mutation 'mutation_2.txt' with part 'all_1_1_0' reason: 'Code: 10, e.displayText() = DB::Exception: Not found column InsertionDateTime in block. There are only columns: TTLDays (version 21.5.5.12 (official build))'. This error maybe retryable or not. In case of unretryable error, mutation can be killed with KILL MUTATION query (version 21.5.5.12 (official build)) [DB Errorcode=341]

insert into test values (now(), 23);

Ok.

select table, column, type, sum(rows) rows, sum(column_bytes_on_disk) on_disk
from system.parts_columns where column like 'TTLDays'
group by database, table, column, type

table | column  | type  | rows | on_disk
------+---------+-------+------+--------
test  | TTLDays | Int32 |    1 |      86
test  | TTLDays | Int16 |    1 |      84
```
projection_future_part.assign(std::move(projection_parts));
        projection_future_part.name = projection.name;
        projection_future_part.path = future_part.path + "/" + projection.name + ".proj/";
        projection_future_part.part_info = {"all", 0, 0, 0};
        MergeTreeData::MergingParams projection_merging_params;
        projection_merging_params.mode = MergeTreeData::MergingParams::Ordinary;
        if (projection.type == ProjectionDescription::Type::Aggregate)
            projection_merging_params.mode = MergeTreeData::MergingParams::Aggregating;
        
        auto merged_projection_part = mergePartsToTemporaryPart(
            projection_future_part,
            projection.metadata,
            merge_entry,
            holder,
            time_of_merge,
            context,
            space_reservation,
            deduplicate,
            deduplicate_by_columns,
            projection_merging_params,
            new_data_part.get());
        new_data_part->addProjectionPart(projection.name, std::move(merged_projection_part));
    }
    if (chosen_merge_algorithm != MergeAlgorithm::Vertical)
        to.writeSuffixAndFinalizePart(new_data_part, need_sync);
    else
        to.writeSuffixAndFinalizePart(new_data_part, need_sync, &storage_columns, &checksums_gathered_columns);
    return new_data_part;
}
MergeTreeData::MutableDataPartPtr MergeTreeDataMergerMutator::mutatePartToTemporaryPart(