Welcome to the arcane world of multiemployer defined benefit pension plans. “Multis” are often called Taft-Hartley plans, after senators Robert Taft of Ohio and Fred Hartley of New Jersey, co-sponsors of the 1947 Labor-Management Relations Act. Just don’t call them union plans, or risk rankling labor executives, who are quick to explain that these collectively bargained plans are managed by boards of trustees staffed with equal numbers of union and employer representatives. But keep in mind that a typical ten-member Taft-Hartley pension fund board consists of five trustees who are members of the same union and five trustees who likely represent competing employers in the same industry.

A cluster of confounding pension rules and regulations began seriously gumming up the multiemployer pension works when financial markets took a nosedive in 2008. Increased employer bankruptcies and unemployment were heaped onto already stagnating union membership rolls. But even...