Example:

db.c.find({}, {easyToOptimize: {$add: [1,2]}}).explain() 

The explain output shows an unoptimized projection:

	"queryPlanner" : {
		"plannerVersion" : 1,
		"namespace" : "test.c",
		"indexFilterSet" : false,
		"parsedQuery" : {
			
		},
		"queryHash" : "27732203",
		"planCacheKey" : "27732203",
		"winningPlan" : {
			"stage" : "PROJECTION_DEFAULT",
			"transformBy" : {
				"f" : {
					"$add" : [
						1,
						2
					]
				}
			},
			"inputStage" : {
				"stage" : "COLLSCAN",
				"direction" : "forward"
			}
		},
		"rejectedPlans" : [ ]
	}, 

That is, the $add has not been constant folded.

The cause of this bug is that the ProjectionStage reports the original BSON for the "transformBy" field, rather than a serialized version of the ProjectionExecutor. See here. The actual projection executor used for answering the query does get optimized.

Unfortunately, because of the way $slice and positional projection are implemented using "root replacement" expressions, serializing the executor back to MQL may not always be possible with the code in its current state.