Sometimes, the soundness of a position only becomes apparent by comparison with the alternative. So it has been, for me, in the gun registry litigation, which has finally concluded this morning with the Supreme Court’s decision in Quebec (Attorney General) v. Canada (Attorney General), 2015 SCC 14. The majority finds that contrary to Québec’s claims, the federal government was entitled to destroy the long gun registration data it had accumulated while registering such guns was mandatory, regardless of provincial objections. The dissenters, for their part, would have held that the statutory provision requiring the destruction of the data is unconstitutional ― but that Québec is not constitutionally entitled to the data, whose fate must be settled by a political negotiation. The dissenters present their position as the more realistic one ― yet their proposed remedy is, in my view, so unpragmatic as to show that this position is unsound.
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The majority, in terse reasons by Justices Cromwell and Karakatsanis, with whom the Chief Justice and Justices Rothstein and Moldaver agree, mostly relies on the principles of federalism ― understood, primarily, as a clear division of legislative powers ― and parliamentary sovereignty. The principle of co-operative federalism, which Québec invoked, can it the majority’s view serve
to provide flexibility in separation of powers doctrines, such as federal paramountcy and interjurisdictional immunity. It is used to facilitate interlocking federal and provincial legislative schemes and to avoid unnecessary constraints on provincial legislative action. 
But it neither amounts to a self-standing constraint on the exercise of legislative powers by either the federal Parliament or the provincial legislatures, nor “impose[s] a positive obligation to facilitate cooperation where the constitutional division of powers authorizes unilateral action.”  Furthermore, the province cannot invoke its reliance on or expectation of the continued existence of the gun registry. Parliament must remain free to abolish what it had previously created. The majority also concludes that provincial participation in the operation of the registry was limited and merely a part of the framework established by federal law.
The majority holds that the provision of the legislation abolishing the long gun registry which requires the destruction of the data was within Parliament’s legislative competence. As the creation of the registry was valid legislation in relation to criminal law, so must be its abolition, and so also must be provision settling the question of what is to happen to the now-unneeded data. The reasons that motivated Parliament to settle that question in the particular way it did rather than otherwise are irrelevant. Nor does a desire to make life more complicated, at a practical level, for the other level of government translate into a constitutionally cognizable harm:
[a]n intention on the part of one level of government to prevent another from realizing a policy objective it disagrees with does not, on its own, lead to the conclusion that there is an encroachment on the other level of government’s sphere of exclusive jurisdiction. 
Since Québec could not “validly enact legislation that deals with what will happen with the data of the repealed scheme,”  Parliament’s enactment of such legislation does not impede the exercise of any provincial powers.
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In contrast to the majority’s, the dissenting reasons, jointly written by Justices Lebel, Wagner, and Gascon, with the agreement of Justice Abella, try to convey the impression of careful attention to the real-life background of the dispute. The dissent starts with a history of gun control in Canada, and includes repeated reference to the Polytechnique Massacre and the importance of gun control to Québec. It emphasizes the statements of various federal politicians (including the future prime minister Stephen Harper) and officials regarding the collaborative nature of the Canadian gun control regime. It also delves deeply into the mechanics of that regime’s operation, and details the ways in which provincial officials contribute information to and rely on its linchpin, the Canadian Firearms Information System database, highlighting the fact that, although legislation suggests that provincially- and federally-collected data ought to be distinct, they are, in reality, very much intertwined.
This leads the dissent to the conclusion that the long gun registry, as part of the broader gun control regime, is the fruit of a “partnership” between the federal government and Québec. This partnership “is consistent with the spirit of co‑operative federalism,” having
enabled the federal and provincial governments to work together, rather than in isolation, to achieve both federal (criminal law) and provincial (public safety and administration of justice) purposes. 
Upholding the principle of (co-operative federalism) thus requires “protect[ing]” such partnerships
both when they are implemented and when they are dismantled. It would hardly make sense to encourage co‑operation and find that schemes established in the context of a partnership are valid while at the same time refusing to take this particular context into account when those schemes are terminated. 
A legislature that wants to dismantle a partnership must “tak[e] into account the reasonably foreseeable consequences of the decision to do so for the other partner”  ― and so must the courts, when they consider the constitutionality of the resulting legislation.
The dissent’s final approach to the issue in the case still takes the form of a traditional division of powers analysis. It concludes that the “true purpose” of the provision requiring the destruction of the gun registry data without offering it to the provinces “is to ensure that the information on long guns can no longer be used for any provincial purposes.”  But because registration falls, fundamentally, under the provincial power over property and civil rights, this provision does too, and is thus, in “pith and substance,” not valid federal legislation in relation to criminal law. Nor can it be justified as ancillary to the broader, valid, exercise of the criminal law power resulting in the abolition of the registry. For one thing, it is not necessary, in order to abolish the federal registry, to refuse to hand over the data to the provinces; for another, in the context of the Canada-Québec “partnership,” the encroachment on the provincial powers is too serious to be justified as ancillary.
The dissent accordingly concludes that the provision at issue is unconstitutional, being beyond Parliament’s legislative powers. However, it also finds that Québec “has not established a legal basis for its claim to the data.” It is not the courts’ role to order the federal government to hand over the gun registry data. Rather, as the data are the “fruit” of a partnership, it is for the partners themselves to decide, as a political matter, how to dispose of them. The province, no more than the federal government, cannot make that decision unilaterally. Since they failed to make that decision at the moment of creating the partnership, they must do so now, by means of a negotiation.
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As I mentioned at the outset, I believe that the majority has the better of the argument here. I find its argument that, since the province could not itself decide what is to happen to the data, the data’s destruction cannot be frustrating the exercise of a provincial legislative power compelling. By contrast, the dissent’s claim that Parliament is “encroaching” on provincial jurisdiction strikes me as unpersuasive. All that Parliament does is to make an eventual exercise of the provincial legislative power more costly (financially and, perhaps, though unlikely in this case, politically). The province is still free to act, so long as it is willing to pay the price. That might not be nice of Parliament but, as I will presently explain, I don’t think that Parliament is under a duty to be nice here.
Before doing so, I want to return to my point about the dissent’s proposed remedy. As I wrote after the oral argument, where Justice Wagner had floated his and his colleagues’ eventual proposed solution,
[d]eclaring a duty to negotiate might have worked (hypothetically of course) in the secession context, where the end point of a successful negotiation would be clear (i.e. a peaceful separation), although the details would need to be worked out. Here, the parties have an irreconcilable disagreement over what the end state would be (data handover or data destruction). What can they negotiate about?
It is striking that, at the conclusion of an opinion that presents itself as attuned to the political realities, a pragmatic rebuttal to a rather dogmatic majority judgment the dissenters propose a remedy that would utterly fail to solve the dispute before it and result in protracted, acrimonious, and ultimately futile negotiations. This is not pragmatism, but either naïveté or a rather cynical bet that, as the negotiations drag on and on, the current federal government will be replaced by one more favourable to Québec’s claims later this year. More importantly though, in my view, the dissent’s proposed remedy amounts to an acknowledgement that Québec’s claim was ultimately political, not legal. “Co-operative federalism” was an attempt to bridge the gap between the two, but even for the dissent, it was not enough.
Still, doesn’t the idea have some value? Shouldn’t there be, in a federation, a sense of good-neighbourliness preventing the two orders of government from exercising their powers in a manner which, though superficially legitimate, imposes serious costs ― something like the neighbourhood disturbances or nuisances of private law ― on each other? Well, perhaps. But there is an important requirement of good-neighbourliness that the majority opinion actually serves much better than the dissent. The majority’s reasoning means that both the federal government and the provinces are clear about their own, and the other’s, entitlements when they embark on a joint venture. If they go ahead, they know the risks. The dissent’s reasoning would only have served to generate confusion about who may do what, and under what circumstances. It is good fences that make good neighbours.