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
```

if (if_exists)
                    return;
                throw Exception("Wrong index name. Cannot find index " + backQuote(index_name) + " to drop.", ErrorCodes::BAD_ARGUMENTS);
            }
            metadata.secondary_indices.erase(erase_it);
        }
    }
    else if (type == ADD_CONSTRAINT)
    {
        if (std::any_of(
                metadata.constraints.constraints.cbegin(),
                metadata.constraints.constraints.cend(),
                [this](const ASTPtr & constraint_ast)
                {
                    return constraint_ast->as<ASTConstraintDeclaration &>().name == constraint_name;
                }))
        {
            if (if_not_exists)
                return;
            throw Exception("Cannot add constraint " + constraint_name + ": constraint with this name already exists",
                        ErrorCodes::ILLEGAL_COLUMN);
        }
        auto insert_it = metadata.constraints.constraints.end();
        metadata.constraints.constraints.emplace(insert_it, std::dynamic_pointer_cast<ASTConstraintDeclaration>(constraint_decl));
    }
    else if (type == DROP_CONSTRAINT)
    {
        auto erase_it = std::find_if(
                metadata.constraints.constraints.begin(),
                metadata.constraints.constraints.end(),