Case C-300/15: Judgment of the Court (Tenth Chamber) of 26 May 2016 (request for a preliminary ruling from the Tribunal administratif — Luxembourg) — Charles Kohll, Sylvie Kohll-Schlesser v Directeur de l’administration des contributions directes (Reference for a preliminary ruling — Articles 21 and 45 TFEU — Freedom of movement and of residence of persons and workers — Income tax — Retirement pension — Pensioners’ tax credit — Conditions for granting — Possession of a tax deduction form issued by national authorities)