Article 49 TFEU (freedom of establishment) is to be interpreted as precluding the application of the tax legislation of a Member State, such as that at issue in the main proceedings, which has the effect that a couple residing in that Member State and earning income both in that Member State and in another Member State does not in fact receive a specific tax advantage, owing to the rules for offsetting it, whereas that couple would receive that tax advantage if the member of the couple earning the higher income did not earn his entire income in another Member State.