The other day I experienced a flash of alarm. There was a claim from an Argentine journalist that when the president of Argentina, Mauricio Macri, phoned Donald Trump to congratulate him on his election victory, the talk turned to permits for the building of a Trump skyscraper in Buenos Aires. Mr. Macri’s press officer quickly and sharply denied the report: “They didn’t talk about the tower at all. It’s absolutely untrue.” So did the Trump transition office. The journalist apparently offered no proof. The story more or less ended there.

But...