fix: finalize flat expression direct builder review notes

Agent-Logs-Url: https://github.com/dadhi/FastExpressionCompiler/sessions/15dd11ed-8bf0-4fd4-b514-3d2ff4e4e346

Co-authored-by: dadhi <39516+dadhi@users.noreply.github.com>

Author: Copilot

src/FastExpressionCompiler.LightExpression/FlatExpression.cs

@@ -783,18 +783,18 @@ ExpressionType.Equal or ExpressionType.NotEqual or ExpressionType.GreaterThan or
         };
     }
 
-    private static Type GetArrayElementType(Type arrayType, int rank)
+    private static Type GetArrayElementType(Type arrayType, int depth)
     {
         var elementType = arrayType;
-        for (var i = 0; i < rank; ++i)
+        for (var i = 0; i < depth; ++i)
             elementType = elementType.GetElementType();
         return elementType ?? typeof(object);
     }
 
     private int CloneChild(int index)
     {
         ref var node = ref Nodes[index];
-        return node.ChildCount == 0
+        return ShouldCloneWhenLinking(node)
             ? AddNode(node.Type, node.Obj, node.NodeType, node.Kind, null, node.ChildIdx)
             : index;
     }
@@ -810,6 +810,14 @@ private int[] CloneChildrenToArray(IEnumerable<int> children)
         return cloned.ToArray();
     }
 
+    // Any leaf reused in more than one parent would have its intrusive sibling link (`NextIdx`) overwritten.
+    // Clone the leaf before linking so the direct builder API may safely reuse returned indexes for
+    // parameters, labels, constants, default values and parameterless `new` expressions.
+    private static bool ShouldCloneWhenLinking(in ExprNode node) =>
+        node.Kind == ExprNodeKind.LabelTarget ||
+        node.NodeType == ExpressionType.Parameter ||
+        node.ChildCount == 0;
+
     private static IEnumerable<int> Single(int item)
     {
         yield return item;

test/FastExpressionCompiler.LightExpression.UnitTests/LightExpressionTests.cs

@@ -473,11 +473,11 @@ public void Can_build_flat_expression_control_flow_directly()
 
             var sysLambda = (System.Linq.Expressions.LambdaExpression)fe.ToExpression();
             var block = (System.Linq.Expressions.BlockExpression)sysLambda.Body;
-            var @goto = (System.Linq.Expressions.GotoExpression)block.Expressions[0];
+            var gotoExpr = (System.Linq.Expressions.GotoExpression)block.Expressions[0];
             var label = (System.Linq.Expressions.LabelExpression)block.Expressions[1];
 
-            Asserts.AreSame(sysLambda.Parameters[0], @goto.Value);
-            Asserts.AreSame(@goto.Target, label.Target);
+            Asserts.AreSame(sysLambda.Parameters[0], gotoExpr.Value);
+            Asserts.AreSame(gotoExpr.Target, label.Target);
         }
 
         public class A