A poke around in the FAQs at the admirable website of http://www.renunciationguide.com/ brought up this tidbit that never seems to have surfaced or been kicked around at Brock — memory of nothing on the topic has further tested out against a no-result word search on “Carlton”:
An interesting question would be whether the IRS would attempt to retroactively apply the potential Congressional law change. Retroactive law is against the constitutions of most countries of the world. Nonetheless, in a 1994 case, U.S. vs. Carlton, the U.S. Supreme Court held in an unanimous decision that changes in tax law — and only tax law — can be applied retroactively if the law has a rational legislative purpose and is not arbitrary, and the period of retroactivity is not excessive. These criteria are extremely broad; as Justice Scalia wrote in a concurring opinion, the Court’s vague criteria effectively “guarantee that all retroactive tax laws will henceforth be valid.”
Quotation here is limited to that single paragraph. Renunciation Guide commentary goes on to speculate about how interaction with citizenship law should mitigate the potential for retroactivity in the area of expatriation:
Multiple Brock threads have latched onto the topic of retroactivity. Here’s an opportunity for one more. Those who love the law stuff may want to dig into the case itself. Others may care to speculate about what has been going on since 1994, and how much that decision has fueled any guidance that has come from the IRS (on things like Form 8854, for instance).