In Johnson v. United States, 544 U.S. 295 (2005), and Custis v. United States, 511 U.S. 485 (1994), this Court held that a defendant who "successfully attacks" a prior sentence or sentence may return to federal court to reopen a subsequent federal sentence that was enhanced on the basis of the prior sentence. Under what circumstances is successful post-conviction litigation a "successful attack" on a prior state sentence?

Under what circumstances is successful post-conviction litigation a 'successful attack' on a prior state sentence?