President Ronald Reagan is invoked incessantly by conservative commentators and politicians, but never more so than when the subject is taxes. And so it was when Donald Trump released his plan for tax reform. Several conservative commentators have done Trump a favor (and the public a disservice) by vouching for the legitimacy of the plan and saying it is very much in the Reagan tradition. That gives Trump far too much credit. His tax plan bears a superficial resemblance to Reagan’s agenda, but Reagan would never have proposed something so transparently infeasible. Reagan is invoked too casually on taxes both...