Creating an index on 3.0 with a partialFilterExpression option (valid in 3.2 or not) should generate a startup warning that explains how the index will contain entries for all documents in the collection. It needs to be dropped and recreated after upgrading to 3.2 in order to actually be a partial index.

If a 3.2 mongod is downgraded to 3.0, then any indexes that have a partialFilterExpression option may not contain index entries for all documents in the collection. We should also generate a startup warning to explain that the index may need to be dropped and recreated to avoid missing documents when it used to answer queries on 3.0.