I tried to rename a key column in a MergeTree table. It does not work, and 20.4 is very clear about the reason. 
20.5.2 is complaining about non-existing column with the old name, suggesting there is a column with the new name:
```
ALTER TABLE rename_key
    RENAME COLUMN key TO new

Received exception from server (version 20.4.6):
Code: 44. DB::Exception: Received from localhost:9000. 
DB::Exception: Trying to ALTER RENAME key key column which is a part of key expression.

Received exception from server (version 20.5.2):
Code: 47. DB::Exception: Received from localhost:9000. 
DB::Exception: Missing columns: 'key' while processing query: 'key', required columns: 'key', source columns: 'value' 'new' 'date'.
```
The message hasn't changed for the partition key:
```
ALTER TABLE rename_key
    RENAME COLUMN date TO new

Received exception from server (version 20.4.6):
Code: 44. DB::Exception: Received from localhost:9000. 
DB::Exception: Trying to ALTER RENAME key date column which is a part of key expression.

Received exception from server (version 20.5.2):
Code: 524. DB::Exception: Received from localhost:9000. 
DB::Exception: Trying to ALTER RENAME key date column which is a part of key expression.
```
The table:
```
CREATE TABLE rename_key
(
    `date` Date,
    `key` UInt64,
    `value` String
)
ENGINE = MergeTree()
PARTITION BY date
ORDER BY key
```

throw Exception(
                                "Cannot drop column " + backQuote(command.column_name) + ", because column " + backQuote(column.name) + " depends on it",
                                ErrorCodes::ILLEGAL_COLUMN);
                    }
                }
                all_columns.remove(command.column_name);
            }
            else if (!command.if_exists)
                throw Exception(
                    "Wrong column name. Cannot find column " + backQuote(command.column_name) + " to drop",
                    ErrorCodes::NOT_FOUND_COLUMN_IN_BLOCK);
        }
        else if (command.type == AlterCommand::COMMENT_COLUMN)
        {
            if (!all_columns.has(command.column_name))
            {
                if (!command.if_exists)
                    throw Exception{"Wrong column name. Cannot find column " + backQuote(command.column_name) + " to comment",
                                    ErrorCodes::NOT_FOUND_COLUMN_IN_BLOCK};
            }
        }
        else if (command.type == AlterCommand::MODIFY_SETTING)
        {
            if (metadata.settings_changes == nullptr)
                throw Exception{"Cannot alter settings, because table engine doesn't support settings changes", ErrorCodes::BAD_ARGUMENTS};
        }
        else if (command.type == AlterCommand::RENAME_COLUMN)
        {
           for (size_t j = i + 1; j < size(); ++j)
           {