SERVER-77280 made it possible for queries like 

coll.find( { $or: [{ _id: 8 }, { a: 5 }] } }) 

to produce a query plan that has both a clustered collection scan and index scan for clustered collections. However, we push down a FETCH stage for each index scan node. We should see if there are cases where we can avoid adding a FETCH stage unnecessarily. One idea is to avoid pushing a FETCH when we have count, like:

coll.find( { $or: [{ _id: 8 }, { a: 5 }] } }).count()