Keeping abreast of Donald Trump’s utterances on tariffs, from actual announcements to vague threats, is a dizzying task. One day he is set on wrecking the integrated North American economy; the next he wants to appease carmakers that depend on it. When it comes to China, he veers between slapping ever-larger levies on its goods and hinting at a desire for a giant trade deal. As for other countries, he speaks ominously of big but so far unspecified tariffs that will soon kick in.