Ok, then I need to change the interface and all classes implementing it.
The interface [did not define a return type](https://github.com/doctrine/dbal/blob/master/lib/Doctrine/DBAL/Driver/ResultStatement.php#L49-L55), so I assumed `void`.

And most of the implementing classes where returning void:

* [Cache\ArrayStatement](https://github.com/doctrine/dbal/blob/master/lib/Doctrine/DBAL/Cache/ArrayStatement.php#L50-L57)
* [Cache\ResultCacheStatement](https://github.com/doctrine/dbal/blob/master/lib/Doctrine/DBAL/Cache/ResultCacheStatement.php#L135-L138)
* [Driver\IBMDB2\DB2Statement](https://github.com/doctrine/dbal/blob/master/lib/Doctrine/DBAL/Driver/IBMDB2/DB2Statement.php#L151-L154)
* [Driver\Mysqli\MysqliStatement](https://github.com/doctrine/dbal/blob/master/lib/Doctrine/DBAL/Driver/Mysqli/MysqliStatement.php#L329-L332)
* [Portability\Statement](https://github.com/doctrine/dbal/blob/master/lib/Doctrine/DBAL/Portability/Statement.php#L104-L108)
* ...