Currently, SBE plan for query

> db.c.createIndex({a:1, b: 1})
> db.c.find({a: {$gt: 3}}, {_id: 0, a: 1, c: {$add: ["$a", "$b"]}}).explain()

Will create an intermediate object, then extract values for fields a and b, compute expression and create final object. We can skip intermediate object and compute expressions directly using slots from index scan. Same strategy is applied for inclusion projections in SERVER-54745.