Fix #886: use default instead of null for optional struct ByRef parameters

When a VB optional ByRef parameter has a struct type with `Nothing` as default,
Roslyn returns null from ExplicitDefaultValue. Generating a null literal causes
a C# compile error since structs cannot be null. Use `default` instead when the
parameter type is a value type and the default value is null.

https://claude.ai/code/session_01AkwUvu3XuCdj3D4axoX4UX

Author: claude

CodeConverter/CSharp/ArgumentConverter.cs

@@ -226,10 +226,10 @@ private CSSyntax.ArgumentSyntax CreateOptionalRefArg(IParameterSymbol p, RefKind
         var type = CommonConversions.GetTypeSyntax(p.Type);
         CSSyntax.ExpressionSyntax initializer;
         if (p.HasExplicitDefaultValue) {
-            initializer = CommonConversions.Literal(p.ExplicitDefaultValue);
+            initializer = LiteralOrDefault(p.ExplicitDefaultValue, p.Type);
         } else if (HasOptionalAttribute(p)) {
             if (TryGetDefaultParameterValueAttributeValue(p, out var defaultValue)) {
-                initializer = CommonConversions.Literal(defaultValue);
+                initializer = LiteralOrDefault(defaultValue, p.Type);
             } else {
                 initializer = CS.SyntaxFactory.DefaultExpression(type);
             }
@@ -273,6 +273,18 @@ bool TryGetDefaultParameterValueAttributeValue(IParameterSymbol p, out object de
         }
     }
 
+    /// <summary>
+    /// Returns a literal expression for the given value, or a <c>default</c> expression when the value is null
+    /// and the parameter type is a value type (e.g. a struct), since null is not a valid C# initializer for value types.
+    /// </summary>
+    private static CSSyntax.ExpressionSyntax LiteralOrDefault(object value, ITypeSymbol paramType)
+    {
+        if (value is null && paramType.IsValueType) {
+            return ValidSyntaxFactory.DefaultExpression;
+        }
+        return CommonConversions.Literal(value);
+    }
+
     public CSSyntax.ArgumentListSyntax CreateArgList(ISymbol invocationSymbol)
     {
         return CS.SyntaxFactory.ArgumentList(CS.SyntaxFactory.SeparatedList(

Tests/CSharp/ExpressionTests/ByRefTests.cs

@@ -914,4 +914,41 @@ public static void LogAndReset(ref int arg)
 }");
     }
 
+    [Fact]
+    public async Task OptionalStructRefParameterUsesDefaultNotNullIssue886Async()
+    {
+        await TestConversionVisualBasicToCSharpAsync(@"Public Class Issue886
+    Private Shared Sub OptionalParams()
+        FunctionWithOptionalParams()
+    End Sub
+
+    Private Shared Sub FunctionWithOptionalParams(Optional ByRef structParam As TestStruct = Nothing)
+        structParam = New TestStruct
+    End Sub
+
+    Friend Structure TestStruct
+        Friend A As Boolean
+    End Structure
+End Class", @"using System.Runtime.InteropServices;
+
+public partial class Issue886
+{
+    private static void OptionalParams()
+    {
+        TestStruct argstructParam = default;
+        FunctionWithOptionalParams(structParam: ref argstructParam);
+    }
+
+    private static void FunctionWithOptionalParams([Optional] ref TestStruct structParam)
+    {
+        structParam = new TestStruct();
+    }
+
+    internal struct TestStruct
+    {
+        internal bool A;
+    }
+}");
+    }
+
 }