International Business Machines Corp. v. Michigan Department of Treasury

(1) Whether a state, without violating the constitutional bar against the impairment of contracts, can retroactively withdraw from the Multistate Tax Compact so as to divest taxpayers of benefits under that compact for a period of 6 1/2 years before that withdrawal; and (2) whether, consistent with due process, a state can, by statute, change its tax laws retroactively for a period of more than six years, when the change was not promptly instituted and when the change was designed to increase state tax revenues by overriding a Michigan Supreme Court decision determining taxpayer obligations under prior law.