Case C-109/20: Judgment of the Court (Grand Chamber) of 26 October 2021 (request for a preliminary ruling from the Högsta domstolen — Sweden) — Republiken Polen v PL Holdings Sàrl (Reference for a preliminary ruling — Agreement between the Government of the Kingdom of Belgium and the Government of the Grand Duchy of Luxembourg, of the one part, and the Government of the People’s Republic of Poland, of the other, concerning the reciprocal promotion and protection of investments, signed on 19 May 1987 — Arbitration proceedings — Dispute between an investor from one Member State and another Member State — Arbitration clause provided for in that agreement contrary to EU law — Invalidity — Ad hoc arbitration agreement between the parties to that dispute — Participation in the arbitration proceedings — Tacit expression by that other Member State of its intention to conclude that arbitration agreement — Unlawfulness)