Based on codereview feedback during SERVER-22726, having a class to represent the cross product of "the index is multikey" and "the index has path-level multikey information" would improve the readability of the code in plan_enumerator.cpp, index_catalog_entry.cpp, etc. It would be possible to then fold the currently separate code paths for when the index is multikey but doesn't have path-level info and when the index is multikey and has path-level info.

https://github.com/mongodb/mongo/blob/r3.3.5/src/mongo/db/index/multikey_paths.h#L37-L43