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Operating System:
ALL
Steps To Reproduce:
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Sample data:

db.s.insert([{"a":1}, {"a":1}, {"a":2}, {"a":2}])

db.s.aggregate([ {$group:{"_id":"$a","s":{$push:"$a"}}}, {$group:{"_id":null,"ds":{$push:"$s"}}}, {$project:{"out":{$setUnion:"$ds"}}}])

The expectation is:`out` to be an array [1,2]. But it does not perform any computation in these cases and returns the "ds" array as such.
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Sample data: db.s.insert([{ "a" :1}, { "a" :1}, { "a" :2}, { "a" :2}]) db.s.aggregate([ {$group:{ "_id" : "$a" , "s" :{$push: "$a" }}}, {$group:{ "_id" : null , "ds" :{$push: "$s" }}}, {$project:{ "out" :{$setUnion: "$ds" }}}]) The expectation is:`out` to be an array [1,2]. But it does not perform any computation in these cases and returns the "ds" array as such.

The $setUnion operator doesn't work as stated in the docs. It doesn't work when its input is a variable which is an array of arrays.

related to

SERVER-31991 Allow n-ary aggregation expressions to compute their array of arguments dynamically