What is the test for announcing a rule of law?

"[j]udicial resolution of the instant dispute requires a court to announce a rule of law which will apply to persons other than the litigants here. The rule might well be challenged in the future. It would not be fair, under these circumstances, for us to prevent these parties from litigating the merits of the issue because of a prior judgment between them. [Citation.]" (Moore v. Panish, supra, 32 Cal.3d at p. 540, fn. 5, 186 Cal.Rptr. 475, 652 P.2d 32.)