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
```
{
        UInt64 watch_curr_elapsed = merge_entry->watch.elapsed();
        ProfileEvents::increment(ProfileEvents::MergesTimeMilliseconds, (watch_curr_elapsed - watch_prev_elapsed) / 1000000);
        watch_prev_elapsed = watch_curr_elapsed;
    }
    void operator() (const Progress & value)
    {
        ProfileEvents::increment(ProfileEvents::MergedUncompressedBytes, value.read_bytes);
        if (stage.is_first)
        {
            ProfileEvents::increment(ProfileEvents::MergedRows, value.read_rows);
            ProfileEvents::increment(ProfileEvents::Merge);
        }
        updateWatch();
        merge_entry->bytes_read_uncompressed += value.read_bytes;
        if (stage.is_first)
            merge_entry->rows_read += value.read_rows;
        stage.total_rows += value.total_rows_to_read;
        stage.rows_read += value.read_rows;
        if (stage.total_rows > 0)
        {
            merge_entry->progress.store(
                stage.initial_progress + stage.weight * stage.rows_read / stage.total_rows,
                std::memory_order_relaxed);
        }
    }
};
static bool needSyncPart(size_t input_rows, size_t input_bytes, const MergeTreeSettings & settings)