What is the standard of review for a District Court's determination that a required party is dispensable under Federal Rule of Civil Procedure 19?

QUESTION PRESENTED It is hornbook law that a district court must ordinarily assess its subject-matter jurisdiction based on the facts as they exist when the complaint is filed. A narrow exception to that rule permits a federal court to create subject matter jurisdiction retroactively by dismissing a party in a case incorrectly premised on diversity jurisdiction. However, this Court has emphasized that this authority exists only where the non-diverse party is dispensable under Federal Rule of Civil Procedure 19, and should be exercised “sparingly.” While the existence of this authority is now settled, the Courts of Appeals are deeply divided over the standard of review of a District Court’s dispensability determination, applying standards varying from “abuse of discretion” to “de novo,” depending on which subsection of Rule 19 is at issue and to what extent the District Court’s dispensability determination rests on questions of law or factual findings. The question presented is: What is the standard of review for a District Court’s determination that a required party is dispensable under Federal Rule of Civil Procedure 19? (ii)