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
```
part_info.min_block = parts.front()->info.min_block;
    part_info.max_block = parts.back()->info.max_block;
    part_info.level = max_level + 1;
    part_info.mutation = max_mutation;
    if (parts.front()->storage.format_version < MERGE_TREE_DATA_MIN_FORMAT_VERSION_WITH_CUSTOM_PARTITIONING)
    {
        DayNum min_date = DayNum(std::numeric_limits<UInt16>::max());
        DayNum max_date = DayNum(std::numeric_limits<UInt16>::min());
        for (const auto & part : parts)
        {
            
            
            
            
            DayNum part_min_date;
            DayNum part_max_date;
            MergeTreePartInfo::parseMinMaxDatesFromPartName(part->name, part_min_date, part_max_date);
            min_date = std::min(min_date, part_min_date);
            max_date = std::max(max_date, part_max_date);
        }
        name = part_info.getPartNameV0(min_date, max_date);
    }
    else
        name = part_info.getPartName();
}
void FutureMergedMutatedPart::updatePath(const MergeTreeData & storage, const ReservationPtr & reservation)
{
    path = storage.getFullPathOnDisk(reservation->getDisk()) + name + "/";
}
MergeTreeDataMergerMutator::MergeTreeDataMergerMutator(MergeTreeData & data_, size_t background_pool_size_)