1. db.test.aggregate([{$setWindowFields: {partitionBy: "$key", output: {total: {$sum: "$val"}}}}]) // no project
2. db.test.aggregate([{$setWindowFields: {partitionBy: "$key", output: {total: {$sum: "$val"}}}},{$project: {x: "$val"}}]) // computing project of a field not computed by the window function
3. db.test.aggregate([{$setWindowFields: {partitionBy: "$key", output: {total: {$sum: "$val"}}}},{$project: {total: 1}}]) // non-computing project

> db.test.explain().aggregate([{$setWindowFields: {partitionBy: "$key", output: {total: {$sum: "$val"}}}},{$project: {x: "$total"}}]).queryPlanner.winningPlan.slotBasedPlan.stages
[5] project [s16 = makeBsonObj(MakeObjSpec(["_id", "x" = Arg(0)], Closed), s15, s6)]
[4] project [s15 = makeBsonObj(MakeObjSpec(["total" = Arg(0)], Open), s5, s14)]
[4] project [s14 =
    if exists(s13)
    then doubleDoubleSumFinalize(s13)
    else 0
]
[4] window [s8, s5, s6, s2] [frameFirst[], frameLast[], lowBound{}, highBound{}] [s13 = {init{}, add{aggDoubleDoubleSum(s2)}, remove{}}]
[4] project [s8 =
    let [
        l101.0 = (s1 ?: null)
    ]
    in
        if isArray(l101.0)
        then fail(14, "An expression used to partition cannot evaluate to value of type array")
        else move(l101.0)
]
[3] sort [s7] [asc] [s5, s1, s6, s2]
[3] project [s7 =
    if (isArray(s1) ?: false)
    then (_internalLeast(s1) ?: undefined)
    else (s1 ?: null)
]
[2] project [s6 = getField(s5, "total")]
[2] mkbson s5 s3 [_id, key, val] keep [] true false
[1] scan s3 s4 none none none none none none lowPriority [s1 = key, s2 = val] @"a8bd34e9-7358-4940-babf-ad873c5b306b" true false