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
```
#pragma once
#include <Storages/MergeTree/MergeTreeData.h>
#include <Storages/MutationCommands.h>
#include <atomic>
#include <functional>
#include <Common/ActionBlocker.h>
#include <Storages/MergeTree/TTLMergeSelector.h>
#include <Storages/MergeTree/MergeAlgorithm.h>
#include <Storages/MergeTree/MergeType.h>
#include <Storages/MergeTree/IMergedBlockOutputStream.h>
namespace DB
{
class MergeProgressCallback;
enum class SelectPartsDecision
{
    SELECTED = 0,
    CANNOT_SELECT = 1,
    NOTHING_TO_MERGE = 2,
};
struct FutureMergedMutatedPart
{
    String name;
    UUID uuid = UUIDHelpers::Nil;
    String path;
    MergeTreeDataPartType type;
    MergeTreePartInfo part_info;
    MergeTreeData::DataPartsVector parts;
    MergeType merge_type = MergeType::REGULAR;
    const MergeTreePartition & getPartition() const { return parts.front()->partition; }