The Court of justice delivered on 21 June 2018 its judgment in case C‑1/17 (Petronas Lubricants Italy SpA v Livio Guida), which is about Article 20.2 Brussels I:

« Article 20(2) of Council Regulation (EC) No 44/2001 […] must be interpreted as meaning that, in a situation such as that at issue in the main proceedings, it gives an employer the right to bring, before the court properly seised of the original proceedings brought by an employee, a counter-claim based on a claim-assignment agreement concluded, after the introduction of the original proceedings, between the employer and the original holder of that claim”.

Source: here