fix complex accumulator visitor bug

Author: adelinowona

src/MongoDB.Driver/Linq/Linq3Implementation/Ast/Expressions/AstComplexAccumulatorExpression.cs

@@ -23,15 +23,15 @@ namespace MongoDB.Driver.Linq.Linq3Implementation.Ast.Expressions
     internal sealed class AstComplexAccumulatorExpression : AstAccumulatorExpression
     {
         private readonly AstComplexAccumulatorOperator _operator;
-        private readonly Dictionary<string, AstExpression> _args;
+        private readonly IReadOnlyDictionary<string, AstExpression> _args;
 
-        public AstComplexAccumulatorExpression(AstComplexAccumulatorOperator @operator, Dictionary<string, AstExpression> args)
+        public AstComplexAccumulatorExpression(AstComplexAccumulatorOperator @operator, IReadOnlyDictionary<string, AstExpression> args)
         {
             _operator = @operator;
             _args = Ensure.IsNotNull(args, nameof(args));
         }
 
-        public Dictionary<string, AstExpression> Args => _args;
+        public IReadOnlyDictionary<string, AstExpression> Args => _args;
 
         public override AstNodeType NodeType => AstNodeType.ComplexAccumulatorExpression;
 
@@ -53,7 +53,7 @@ public override BsonValue Render()
             return new BsonDocument(_operator.Render(), document);
         }
 
-        public AstComplexAccumulatorExpression Update(Dictionary<string, AstExpression> args)
+        public AstComplexAccumulatorExpression Update(IReadOnlyDictionary<string, AstExpression> args)
         {
             if (ReferenceEquals(args, _args))
             {

src/MongoDB.Driver/Linq/Linq3Implementation/Ast/Visitors/AstNodeVisitor.cs

@@ -209,9 +209,26 @@ public virtual AstNode VisitComplexAccumulatorExpression(AstComplexAccumulatorEx
 
                 if (newArg != oldArg)
                 {
-                    newArgs ??= new Dictionary<string, AstExpression>(node.Args);
+                    if (newArgs == null)
+                    {
+                        // First change detected - copy all processed entries
+                        newArgs = new Dictionary<string, AstExpression>();
+                        foreach (var processedKvp in node.Args)
+                        {
+                            if (processedKvp.Key == kvp.Key)
+                            {
+                                break; // Stop at current entry
+                            }
+                            newArgs[processedKvp.Key] = processedKvp.Value;
+                        }
+                    }
                     newArgs[kvp.Key] = newArg;
                 }
+                else if (newArgs != null)
+                {
+                    // We're building a new dictionary, so add unchanged entries too
+                    newArgs[kvp.Key] = oldArg;
+                }
             }
 
             return newArgs != null ? node.Update(newArgs) : node;