SERVER-59103 Wrap 'stage' in a stageOrLimitCoScan call to prevent a null stage being passed and add test coverage.

2 files changed, 6 insertions, 1 deletions

diff --git a/jstests/aggregation/expressions/concat_arrays.js b/jstests/aggregation/expressions/concat_arrays.js
index 37f4b6c4a72..924e9037efc 100644
--- a/jstests/aggregation/expressions/concat_arrays.js
+++ b/jstests/aggregation/expressions/concat_arrays.js
@@ -75,6 +75,11 @@ runAndAssert(["$str_arr", {$filter: {input: "$int_arr",
                                 ] }}}, "$int_arr"],
              [["a", "b", "c", 2, 3, 1, 2, 3, 4]]);
 
+// Confirm that empty arrays can be concatenated with variables.
+runAndAssert(
+    ["$str_arr", {$filter: {input: [], cond: {$isArray: [{$concatArrays: [[], "$$this"]}]}}}],
+    [["a", "b", "c"]]);
+
 // Confirm that having any combination of null or missing inputs and valid inputs produces null.
 runAndAssertNull(["$int_arr", "$null_val"]);
 runAndAssertNull(["$int_arr", null]);
diff --git a/src/mongo/db/query/sbe_stage_builder_helpers.cpp b/src/mongo/db/query/sbe_stage_builder_helpers.cpp
index 80fcdf7df0b..71015bef45f 100644
--- a/src/mongo/db/query/sbe_stage_builder_helpers.cpp
+++ b/src/mongo/db/query/sbe_stage_builder_helpers.cpp
@@ -311,7 +311,7 @@ std::pair<sbe::value::SlotId, EvalStage> projectEvalExpr(
     sbe::value::SlotIdGenerator* slotIdGenerator) {
     // If expr's value is already in a slot, return the slot.
     if (expr.getSlot()) {
-        return {*expr.getSlot(), std::move(stage)};
+        return {*expr.getSlot(), stageOrLimitCoScan(std::move(stage), planNodeId)};
     }
 
     // If expr's value is an expression, create a ProjectStage to evaluate the expression