https://github.com/mongodb/mongo/blob/master/src/mongo/db/index/expression_keys_private.cpp#L562 and https://github.com/mongodb/mongo/blob/ec132343b7e7225e36575f173c242b383f505eb2/src/mongo/db/index/expression_keys_private.cpp#L546 needs to be changed from copy to move (or swap).