Is national legislation, such as that at issue, which imposes different treatment for wage purposes on workers with fixed-term contracts within the meaning of clause of Directive 1999/70/EC 1 as compared to the comparable permanent worker, on the basis of the sole criterion of differentiation that their contracts are classified by their employer or by law as fixed-term contracts for work, compatible with clause 4 of Directive 1999/70/EC?
In particular, is national legislation under which different treatment of workers for wage purposes is justified on the ground that they provided their work under fixed-term contracts in the knowledge that they were covering fixed and permanent needs of the employer compatible with clause 4 of Directive 1999/70/EC?