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
```
"Merge sorted {} rows, containing {} columns ({} merged, {} gathered) in {} sec., {} rows/sec., {}/sec.",
            merge_entry->rows_read,
            all_column_names.size(),
            merging_column_names.size(),
            gathering_column_names.size(),
            elapsed_seconds,
            merge_entry->rows_read / elapsed_seconds,
            ReadableSize(merge_entry->bytes_read_uncompressed / elapsed_seconds));
    }
    for (const auto & projection : metadata_snapshot->getProjections())
    {
        MergeTreeData::DataPartsVector projection_parts;
        for (const auto & part : parts)
        {
            auto it = part->getProjectionParts().find(projection.name);
            if (it != part->getProjectionParts().end())
                projection_parts.push_back(it->second);
        }
        if (projection_parts.size() < parts.size())
        {
            LOG_DEBUG(log, "Projection {} is not merged because some parts don't have it", projection.name);
            continue;
        }
        LOG_DEBUG(
            log,
            "Selected {} projection_parts from {} to {}",
            projection_parts.size(),
            projection_parts.front()->name,
            projection_parts.back()->name);
        FutureMergedMutatedPart projection_future_part;