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
```
public :
    /** Is used to cancel all merges and mutations. On cancel() call all currently running actions will throw exception soon.
      * All new attempts to start a merge or mutation will throw an exception until all 'LockHolder' objects will be destroyed.
      */
    ActionBlocker merges_blocker;
    ActionBlocker ttl_merges_blocker;
private:
    MergeAlgorithm chooseMergeAlgorithm(
        const MergeTreeData::DataPartsVector & parts,
        size_t rows_upper_bound,
        const NamesAndTypesList & gathering_columns,
        bool deduplicate,
        bool need_remove_expired_values,
        const MergeTreeData::MergingParams & merging_params) const;
    bool checkOperationIsNotCanceled(const MergeListEntry & merge_entry) const;
private:
    MergeTreeData & data;
    const size_t background_pool_size;
    Poco::Logger * log;
    
    time_t disk_space_warning_time = 0;
    
    ITTLMergeSelector::PartitionIdToTTLs next_delete_ttl_merge_times_by_partition;
    
    ITTLMergeSelector::PartitionIdToTTLs next_recompress_ttl_merge_times_by_partition;
    
    
};
}