A good case could be made that a contract which demands the forceable expropriation of the property of others is null. This is what we’re talking about with government employee pension (salaries, other benefits).
A private company can form a contract with its employees guaranteeing this or that compensation; yet, if the company does not have sufficient income to meet the agreed payments, the employees do not have the legal option of taking the money from non-customers by force. The employees have recourse only against the employer, for whatever they can get.
It's even more simple than that.
Any "contract" which pretends to oblige a future legislature to raise a specific amount of revenue is void on its face.