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diff --git a/src/mongo/db/pipeline/document_source_union_with.h b/src/mongo/db/pipeline/document_source_union_with.h
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+/**
+ *    Copyright (C) 2019-present MongoDB, Inc.
+ *
+ *    This program is free software: you can redistribute it and/or modify
+ *    it under the terms of the Server Side Public License, version 1,
+ *    as published by MongoDB, Inc.
+ *
+ *    This program is distributed in the hope that it will be useful,
+ *    but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *    Server Side Public License for more details.
+ *
+ *    You should have received a copy of the Server Side Public License
+ *    along with this program. If not, see
+ *    <http://www.mongodb.com/licensing/server-side-public-license>.
+ *
+ *    As a special exception, the copyright holders give permission to link the
+ *    code of portions of this program with the OpenSSL library under certain
+ *    conditions as described in each individual source file and distribute
+ *    linked combinations including the program with the OpenSSL library. You
+ *    must comply with the Server Side Public License in all respects for
+ *    all of the code used other than as permitted herein. If you modify file(s)
+ *    with this exception, you may extend this exception to your version of the
+ *    file(s), but you are not obligated to do so. If you do not wish to do so,
+ *    delete this exception statement from your version. If you delete this
+ *    exception statement from all source files in the program, then also delete
+ *    it in the license file.
+ */
+
+#pragma once
+
+#include <boost/optional.hpp>
+
+#include "mongo/db/pipeline/document_source.h"
+#include "mongo/db/pipeline/lite_parsed_pipeline.h"
+
+namespace mongo {
+
+class DocumentSourceUnionWith final : public DocumentSource {
+public:
+    static constexpr StringData kStageName = "$unionWith"_sd;
+
+    static boost::intrusive_ptr<DocumentSource> createFromBson(
+        BSONElement elem, const boost::intrusive_ptr<ExpressionContext>& expCtx);
+
+    class LiteParsed final : public LiteParsedDocumentSource {
+    public:
+        static std::unique_ptr<LiteParsed> parse(const AggregationRequest& request,
+                                                 const BSONElement& spec);
+
+        LiteParsed(NamespaceString withNss,
+                   stdx::unordered_set<NamespaceString> foreignNssSet,
+                   boost::optional<LiteParsedPipeline> liteParsedPipeline)
+            : _withNss{std::move(withNss)},
+              _foreignNssSet(std::move(foreignNssSet)),
+              _liteParsedPipeline(std::move(liteParsedPipeline)) {}
+
+        stdx::unordered_set<NamespaceString> getInvolvedNamespaces() const final {
+            return {_foreignNssSet};
+        }
+
+        PrivilegeVector requiredPrivileges(bool) const final {
+            return {};
+        }
+
+        bool allowShardedForeignCollection(NamespaceString) const final {
+            return true;
+        }
+
+        bool allowedToPassthroughFromMongos() const final {
+            return true;
+        }
+
+    private:
+        const NamespaceString _withNss;
+        const stdx::unordered_set<NamespaceString> _foreignNssSet;
+        const boost::optional<LiteParsedPipeline> _liteParsedPipeline;
+    };
+
+    DocumentSourceUnionWith(const boost::intrusive_ptr<ExpressionContext>& expCtx,
+                            std::unique_ptr<Pipeline, PipelineDeleter> pipeline)
+        : DocumentSource(kStageName, expCtx), _pipeline(std::move(pipeline)) {}
+
+    const char* getSourceName() const final {
+        return kStageName.rawData();
+    }
+
+    void serializeToArray(
+        std::vector<Value>& array,
+        boost::optional<ExplainOptions::Verbosity> explain = boost::none) const final;
+
+    GetModPathsReturn getModifiedPaths() const final;
+
+    StageConstraints constraints(Pipeline::SplitState) const final {
+        return StageConstraints(StreamType::kStreaming,
+                                PositionRequirement::kNone,
+                                HostTypeRequirement::kNone,
+                                DiskUseRequirement::kNoDiskUse,
+                                FacetRequirement::kAllowed,
+                                TransactionRequirement::kNotAllowed,
+                                LookupRequirement::kNotAllowed);
+    }
+
+    DepsTracker::State getDependencies(DepsTracker* deps) const final;
+
+    boost::optional<DistributedPlanLogic> distributedPlanLogic() final {
+        // {shardsStage, mergingStage, sortPattern}
+        return DistributedPlanLogic{nullptr, this, boost::none};
+    }
+
+    void addInvolvedCollections(stdx::unordered_set<NamespaceString>* collectionNames) const final;
+
+    void detachFromOperationContext() final;
+
+    void reattachToOperationContext(OperationContext* opCtx) final;
+
+protected:
+    GetNextResult doGetNext() final;
+    void doDispose() final;
+
+private:
+    /**
+     * Should not be called; use serializeToArray instead.
+     */
+    Value serialize(boost::optional<ExplainOptions::Verbosity> explain = boost::none) const final {
+        MONGO_UNREACHABLE;
+    }
+
+    std::unique_ptr<Pipeline, PipelineDeleter> _pipeline;
+    bool _sourceExhausted = false;
+    bool _cursorAttached = false;
+};
+
+}  // namespace mongo