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
```
{
    if (parts_.empty())
        return;
    for (const MergeTreeData::DataPartPtr & part : parts_)
    {
        const MergeTreeData::DataPartPtr & first_part = parts_.front();
        if (part->partition.value != first_part->partition.value)
            throw Exception(
                "Attempting to merge parts " + first_part->name + " and " + part->name + " that are in different partitions",
                ErrorCodes::LOGICAL_ERROR);
    }
    parts = std::move(parts_);
    UInt32 max_level = 0;
    Int64 max_mutation = 0;
    for (const auto & part : parts)
    {
        max_level = std::max(max_level, part->info.level);
        max_mutation = std::max(max_mutation, part->info.mutation);
    }
    type = future_part_type;
    part_info.partition_id = parts.front()->info.partition_id;