AddParameter should handle and bail out on omitted argument (#23933)

src/EditorFeatures/VisualBasicTest/AddParameter/AddParameterTests.vb

@@ -13,6 +13,25 @@ Namespace Microsoft.CodeAnalysis.Editor.VisualBasic.UnitTests.AddParameter
             Return (Nothing, New VisualBasicAddParameterCodeFixProvider())
         End Function
 
+        <Fact, Trait(Traits.Feature, Traits.Features.CodeActionsAddParameter)>
+        <WorkItem(23927, "https://github.com/dotnet/roslyn/issues/23927")>
+        Public Async Function TestMissingOnOmittedArgument() As Task
+            Await TestMissingAsync(
+"Public Module Module1
+    Private Class C
+        Public Sub New(Arg1 As Integer)
+        End Sub
+
+        Public Sub New(Arg1 As Integer, Arg2 As Integer)
+        End Sub
+    End Class
+
+    Public Sub Main()
+        Dim x = New [|C|](, 0)
+    End Sub
+End Module")
+        End Function
+
         <Fact, Trait(Traits.Feature, Traits.Features.CodeActionsAddParameter)>
         Public Async Function TestMissingWithImplicitConstructor() As Task
             Await TestMissingAsync(

src/Features/Core/Portable/AddParameter/AbstractAddParameterCodeFixProvider.cs

@@ -442,10 +442,14 @@ private int NonParamsParameterCount(IMethodSymbol method)
 
                     // Now check the type of the argument versus the type of the parameter.  If they
                     // don't match, then this is the argument we should make the parameter for.
-                    var expressionOfArgumment = syntaxFacts.GetExpressionOfArgument(argument);
-                    var argumentTypeInfo = semanticModel.GetTypeInfo(expressionOfArgumment);
-                    var isNullLiteral = syntaxFacts.IsNullLiteralExpression(expressionOfArgumment);
-                    var isDefaultLiteral = syntaxFacts.IsDefaultLiteralExpression(expressionOfArgumment);
+                    var expressionOfArgument = syntaxFacts.GetExpressionOfArgument(argument);
+                    if (expressionOfArgument is null)
+                    {
+                        return null;
+                    }
+                    var argumentTypeInfo = semanticModel.GetTypeInfo(expressionOfArgument);
+                    var isNullLiteral = syntaxFacts.IsNullLiteralExpression(expressionOfArgument);
+                    var isDefaultLiteral = syntaxFacts.IsDefaultLiteralExpression(expressionOfArgument);
 
                     if (argumentTypeInfo.Type == null && argumentTypeInfo.ConvertedType == null)
                     {