Expose output of `SetWindowFieldsOperation` correctly to next aggregation stage.

This commit makes sure to expose calculated output fields correctly.

Original pull request #4751
Closes #4745

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/SetWindowFieldsOperation.java

@@ -70,7 +70,7 @@ public class SetWindowFieldsOperation
 
 	@Override
 	public ExposedFields getFields() {
-		return ExposedFields.nonSynthetic(Fields.from(output.fields.toArray(new Field[0])));
+		return ExposedFields.synthetic(Fields.from(output.fields.toArray(new Field[0])));
 	}
 
 	@Override

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/SetWindowFieldsOperationTests.java

@@ -73,6 +73,29 @@ class SetWindowFieldsOperationTests {
 				238, 378);
 	}
 
+	@Test // GH-4745
+	void exposesFieldsToNextStageCorrectly() {
+
+		initCakeSales();
+
+		SetWindowFieldsOperation setWindowFieldsOperation = SetWindowFieldsOperation.builder() //
+				.partitionByField("state") // resolves to field ref "$state"
+				.sortBy(Sort.by(Direction.ASC, "date")) // resolves to "orderDate"
+				.output(AccumulatorOperators.valueOf("qty").sum()) // resolves to "$quantity"
+				.within(Windows.documents().fromUnbounded().toCurrent().build()) //
+				.as("cumulativeQuantityForState") //
+				.build(); //
+
+		AggregationResults<Document> results = mongoTemplate.aggregateAndReturn(Document.class)
+				.by(Aggregation.newAggregation(CakeSale.class, setWindowFieldsOperation,
+						/* and now project on the field to see it can be referenced */
+						Aggregation.project("cumulativeQuantityForState")))
+				.all();
+
+		assertThat(results.getMappedResults()).map(it -> it.get("cumulativeQuantityForState")).contains(162, 282, 427, 134,
+				238, 378);
+	}
+
 	@Test // GH-3711
 	void executesSetWindowFieldsOperationWithPartitionExpressionCorrectly() {