Add arglist test and revert changes to BoundByRefArgumentWithCopyBack as it is...

Add arglist test and revert changes to BoundByRefArgumentWithCopyBack as it is never exposed in the operation tree

src/Compilers/CSharp/Portable/BoundTree/Expression.cs

@@ -1584,6 +1584,8 @@ internal partial class BoundArgListOperator
     {
         protected override OperationKind ExpressionKind => OperationKind.None;
 
+        protected override ImmutableArray<IOperation> Children => Arguments.As<IOperation>();
+
         public override void Accept(OperationVisitor visitor)
         {
             visitor.VisitNoneOperation(this);

src/Compilers/CSharp/Test/Semantic/IOperation/IOperationTests_IParameterReferenceExpression.cs

@@ -706,5 +706,32 @@ public I F(object x)
 
             VerifyOperationTreeAndDiagnosticsForTest<ObjectCreationExpressionSyntax>(compilation1, expectedOperationTree, expectedDiagnostics);
         }
+
+        [Fact, WorkItem(8884, "https://github.com/dotnet/roslyn/issues/8884")]
+        public void ParameterReference_ArgListOperator()
+        {
+            string source = @"
+using System;
+class C
+{
+    static void Method(int x, bool y)
+    {
+        M(1, /*<bind>*/__arglist(x, y)/*</bind>*/);
+    }
+    
+    static void M(int x, __arglist)
+    {
+    } 
+}
+";
+            string expectedOperationTree = @"
+IOperation:  (OperationKind.None) (Syntax: '__arglist(x, y)')
+  Children(2): IParameterReferenceExpression: x (OperationKind.ParameterReferenceExpression, Type: System.Int32) (Syntax: 'x')
+    IParameterReferenceExpression: y (OperationKind.ParameterReferenceExpression, Type: System.Boolean) (Syntax: 'y')
+";
+            var expectedDiagnostics = DiagnosticDescription.None;
+
+            VerifyOperationTreeAndDiagnosticsForTest<InvocationExpressionSyntax>(source, expectedOperationTree, expectedDiagnostics);
+        }
     }
 }
\ No newline at end of file

src/Compilers/VisualBasic/Portable/BoundTree/Expression.vb

@@ -2426,12 +2426,6 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
             Return OperationKind.None
         End Function
 
-        Protected Overrides ReadOnly Property Children As ImmutableArray(Of IOperation)
-            Get
-                Return ImmutableArray.Create(Of IOperation)(Me.OriginalArgument)
-            End Get
-        End Property
-
         Public Overrides Sub Accept(visitor As OperationVisitor)
             visitor.VisitNoneOperation(Me)
         End Sub