In practice, however, friendly suits are rarely explicitly described as such, and they could easily slip into the federal judicial system through some casual omissions.
Moreover, the "case or controversy" requirement of Article III does not bind the judiciaries of the states, which are free to impose their own restrictions on friendly suits (or none at all).
For example, if two people think a law is unconstitutional, one might sue another in order to put the lawsuit before a court which can rule on its constitutionality.
[3] Originally stated in Chicago & Grand Trunk Railway Company v. Wellman (1892),[4] the collusive lawsuit rule is one the seven rules of the constitutional avoidance doctrine established in Ashwander v. Tennessee Valley Authority (1936) that requires that the Supreme Court of the United States to "not [rule] upon the constitutionality of legislation in a friendly, nonadversary, proceeding, declining because to decide such questions 'is legitimate only in the last resort, and as a necessity in the determination of real, earnest and vital controversy between individuals.
It never was the thought that, by means of a friendly suit, a party beaten in the legislature could transfer to the courts an inquiry as to the constitutionality of the legislative act.