In recent years, federal courts have been addressing corporate class-action defendants' claims that the federal class-action rule (Rule 23) contains an rigorous "ascertainability" requirement — a supposed need for the plaintiffs to show, at the certification stage (rather than at the remedy phase), that they have a crackerjack method for identifying the class members with precision. Corporate defendants and their trade groups have asked the Supreme Court to take on this issue in the recent past, but without success so far.
The latest attempt came in May of this year in Conagra v. Briseno, No. 16-1221 (cert-stage briefs here). The Supreme Court will decide whether to review the case sometime this fall (probably in October).