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
```
FutureMergedMutatedPart() = default;
    explicit FutureMergedMutatedPart(MergeTreeData::DataPartsVector parts_)
    {
        assign(std::move(parts_));
    }
    FutureMergedMutatedPart(MergeTreeData::DataPartsVector parts_, MergeTreeDataPartType future_part_type)
    {
        assign(std::move(parts_), future_part_type);
    }
    void assign(MergeTreeData::DataPartsVector parts_);
    void assign(MergeTreeData::DataPartsVector parts_, MergeTreeDataPartType future_part_type);
    void updatePath(const MergeTreeData & storage, const ReservationPtr & reservation);
};
/** Can select parts for background processes and do them.
 * Currently helps with merges, mutations and moves
 */
class MergeTreeDataMergerMutator
{
public:
    using AllowedMergingPredicate = std::function<bool (const MergeTreeData::DataPartPtr &, const MergeTreeData::DataPartPtr &, String *)>;
    MergeTreeDataMergerMutator(MergeTreeData & data_, size_t background_pool_size);
    /** Get maximum total size of parts to do merge, at current moment of time.
      * It depends on number of free threads in background_pool and amount of free space in disk.
      */
    UInt64 getMaxSourcePartsSizeForMerge() const;