For sharded collection, the following query gives incorrect results:

coll.aggregate([{$group: {_id: null, result: {$topN: {n: 3, output: "abc", sortBy: {}}}}}])

This is a pretty useless query. However, the results from both Classic engine and SBE are incorrect:

{
	"_id" : null,
	"result" : [
		{
			"output" : "abc",
			"sortFields" : [ ]
		},
		{
			"output" : "abc",
			"sortFields" : [ ]
		},
		{
			"output" : "abc",
			"sortFields" : [ ]
		}
	]
}

This is due to the erroneous logic for serializing constant argument: https://github.com/10gen/mongo/blob/c6b787ec1d8cc77b3ae09fda85e278cd6e752ee6/src/mongo/db/pipeline/accumulator_multi.cpp#L473

When the argument is constant, the serialized value will be {$const: {output: ...}} instead of

, so the logic to distinguish merging group is wrong and the output field is appended incorrectly.