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
```

return {};
}
void AlterCommand::apply(StorageInMemoryMetadata & metadata, const Context & context) const
{
    if (type == ADD_COLUMN)
    {
        ColumnDescription column(column_name, data_type);
        if (default_expression)
        {
            column.default_desc.kind = default_kind;
            column.default_desc.expression = default_expression;
        }
        if (comment)
            column.comment = *comment;
        column.codec = codec;
        column.ttl = ttl;
        metadata.columns.add(column, after_column, first);
        
        metadata.columns.flattenNested();
    }
    else if (type == DROP_COLUMN)
    {
        
        if (!clear && !partition)
            metadata.columns.remove(column_name);
    }
    else if (type == MODIFY_COLUMN)
    {
        metadata.columns.modify(column_name, after_column, first, [&](ColumnDescription & column)
        {