A checked conversion to a union of object types tests each member's
fields with the members' matchers. For a field of type `C | undefined`
holding a class instance, the union's matcher found the instance's
capsule, which is not a capsule of the union itself, and answered no
(or matched the instance's plain-object view, which no class arm
accepts), so no member matched and the conversion threw a TypeError.
workerData read as such a union failed this way in every worker. A union
now matches capsules of other types through its arms, where the class
arm accepts the instance.
Chris Tate
064b072f94