When you run the same two tests against the 4.3 branch, the exception goes as follows:

HV000039: Invalid property path. There is no property foo in entity java.util.HashSet

From what I understood, the path iterator for a path list[0].foo does not seem to go list -> list[0] -> list[0].foo, but it goes to list[0] right away, while the metadata still return the List, not the list element.