Canada (Minister of Citizenship and Immigration) v Vavilov

Canada (Minister of Citizenship and Immigration) v Vavilov, 2019 SCC 65, is a decision of the Supreme Court of Canada that revised the standard of review in Canadian administrative law. Legally, "Vavilov" established a presumption that reasonableness is the applicable standard of review in most cases. Factually, the decision allowed Alexander Vavilov, born in Canada to covert Russian agents, to be a Canadian citizen.
Facts.
"Vavilov" concerns the proper interpretation of a provision of the "Citizenship Act" as applied to Alexander Vavilov. Vavilov was born in Toronto to parents who, it later turned out, were covert Russian agents working under the auspices of the Illegals Program. Their story became the subject of "The Americans".
The question was whether the "Citizenship Act" barred Vavilov from being considered a citizen under the "Citizenship Act", which prevents children of a "diplomatic or consular officer or other representative or employee in Canada of a foreign government" from receiving Canadian citizenship. Canada's Registrar of Citizenship held that the statute did bar Vavilov from receiving citizenship. The Federal Court agreed with the Registrar; Vavilov's appeal to the Federal Court of Appeal was allowed.
Background.
In Canada, before a court assesses whether the decision of an administrative tribunal was lawful or not, it decides what standard of review to apply to that decision. To determine the standard of review, in essence, is to decide how much scrutiny the reviewing court will apply to the decision. 
From the 1980s to the early 2000s, Canadian courts had three standards of review to choose from: patent unreasonableness, under which the reviewing court would only overturn the decision if it was plainly defective; reasonableness simpliciter, under which the reviewing court would determine if the reasons given by the administrative decisionmaker in fact supported its decision; and correctness, in which the reviewing court would substitute its own judgment for the decisionmaker's. 
The "patent unreasonableness" standard was eliminated in "Dunsmuir v New Brunswick", which established two standards of review: reasonableness, a more deferential standard; and correctness, a non-deferential standard. Under "Dunsmuir", a reviewing court would determine which standard applied by applying a multi-part test, which considered, among other things, which standard of review had been applied in the past, and whether the question at issue fell into a set of categories in which correctness review was appropriate.
Supreme Court.
The Supreme Court, in a 343-paragraph judgment, agreed with Vavilov and quashed the Registrar's decision. It determined that the Registrar's decision was unreasonable (in the technical sense described above). Thus, Vavilov was able to regain his Canadian citizenship. The court, in holding for Vavilov, established a new framework for determining the standard of review in Canadian administrative law. 
First, the court decided that reasonableness was the default standard of review. It then outlined two kinds of exceptions to that general rule, under which the correctness standard would apply instead. The first exception is where the legislature has indicated that correctness is appropriate. This may be case where the relevant statute explicitly defines the standard of review, or where the statute allows a litigant to appeal a decision of an administrative tribunal to a court (instead of using judicial review). The second exception is where the rule of law requires a correctness standard. The court held that this will be the case when constitutional questions are at issue, when the administrative decision involves a "general question of law of central importance to the legal system as a whole", or when the decision under review pertains to the jurisdiction of two or more tribunals.