SERVER-57164 isVariableReference includes system variables

This fixes an invariant failure in DocumentSourceGroup.

2 files changed, 17 insertions, 3 deletions

diff --git a/src/mongo/db/pipeline/expression.h b/src/mongo/db/pipeline/expression.h
index 7abd72e8543..74ff64a9991 100644
--- a/src/mongo/db/pipeline/expression.h
+++ b/src/mongo/db/pipeline/expression.h
@@ -1526,12 +1526,26 @@ public:
 
 class ExpressionFieldPath final : public Expression {
 public:
-    bool isRootFieldPath() const {
+    /**
+     * Checks whether this field path is exactly "$$ROOT".
+     */
+    bool isROOT() const {
         return _variable == Variables::kRootId && _fieldPath.getPathLength() == 1;
     }
 
+    /**
+     * Checks whether this field path starts with a variable besides ROOT.
+     *
+     * For example, these are variable references:
+     *   "$$NOW"
+     *   "$$NOW.x"
+     * and these are not:
+     *   "$x"
+     *   "$$ROOT"
+     *   "$$ROOT.x"
+     */
     bool isVariableReference() const {
-        return Variables::isUserDefinedVariable(_variable);
+        return _variable != Variables::kRootId;
     }
 
     boost::intrusive_ptr<Expression> optimize() final;
diff --git a/src/mongo/db/pipeline/semantic_analysis.cpp b/src/mongo/db/pipeline/semantic_analysis.cpp
index 36f1985a913..c1613a6a85f 100644
--- a/src/mongo/db/pipeline/semantic_analysis.cpp
+++ b/src/mongo/db/pipeline/semantic_analysis.cpp
@@ -130,7 +130,7 @@ boost::optional<std::string> replaceRootNestsRoot(
     }
     auto&& [nestedName, expression] = children[0];
     if (!dynamic_cast<ExpressionFieldPath*>(expression.get()) ||
-        !dynamic_cast<ExpressionFieldPath*>(expression.get())->isRootFieldPath()) {
+        !dynamic_cast<ExpressionFieldPath*>(expression.get())->isROOT()) {
         return boost::none;
     }
     return nestedName;