I don't know about "illegal" but it seems the Case or Controversy Clause has bearing in that hypothetical. If no actual controversy exists between the parties -

I don't know about "illegal" but it seems the Case or Controversy Clause has bearing in that hypothetical. If no actual controversy exists between the parties - for example, if both sides have agreed on who is to win the case - then the courts generally don't like to issue a ruling. Further, if a litigant fails to raise an issue while arguing a case, and the court therefore doesn't consider it, then the precedent doesn't necessarily control a future case where a litigant does raise that issue.