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

When a VB method has an optional ByRef struct (value type) parameter with
Nothing as the default, the converter previously generated `null` as the
initializer for the hoisted local variable. `null` is not valid for value
types in C#, causing a compilation error.

The fix introduces a `LiteralOrDefault` helper that returns the `default`
literal when the parameter type is a value type and the explicit default
value is null.

https://claude.ai/code/session_01AkwUvu3XuCdj3D4axoX4UX

Author: claude

CodeConverter/CSharp/ArgumentConverter.cs

@@ -97,7 +97,7 @@ CSSyntax.ArgumentSyntax ConvertOmittedArgument(IParameterSymbol parameter)
             var csRefKind = CommonConversions.GetCsRefKind(parameter);
             return csRefKind != RefKind.None
                 ? CreateOptionalRefArg(parameter, csRefKind)
-                : CS.SyntaxFactory.Argument(CommonConversions.Literal(parameter.ExplicitDefaultValue));
+                : CS.SyntaxFactory.Argument(LiteralOrDefault(parameter.ExplicitDefaultValue, parameter.Type));
         }
     }
 
@@ -144,6 +144,14 @@ internal CSSyntax.ExpressionSyntax HoistByRefDeclaration(VBSyntax.ExpressionSynt
         return local.IdentifierName;
     }
 
+    private static CSSyntax.ExpressionSyntax LiteralOrDefault(object value, ITypeSymbol paramType)
+    {
+        if (value is null && paramType.IsValueType) {
+            return ValidSyntaxFactory.DefaultExpression;
+        }
+        return CommonConversions.Literal(value);
+    }
+
     private ISymbol GetInvocationSymbol(SyntaxNode invocation)
     {
         var symbol = invocation.TypeSwitch(
@@ -226,7 +234,7 @@ 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);

Tests/CSharp/ExpressionTests/ByRefTests.cs

@@ -961,4 +961,34 @@ End Sub
         Assert.Contains("GetLicenseMaybe", output);
     }
 
+    [Fact]
+    public async Task OptionalStructRefParameterUsesDefaultNotNullIssue886Async()
+    {
+        // Issue #886: When a VB method has an optional ByRef struct parameter with Nothing as the default,
+        // the generated C# should use `default` rather than `null` (null is not valid for value types).
+        await TestConversionVisualBasicToCSharpAsync(@"
+Structure S
+End Structure
+Sub Foo(Optional ByRef s As S = Nothing)
+End Sub
+Sub Bar()
+    Foo()
+End Sub
+", @"using System.Runtime.InteropServices;
+
+internal partial struct S
+{
+}
+
+public void Foo([Optional] ref S s)
+{
+}
+
+public void Bar()
+{
+    S args = default;
+    Foo(s: ref args);
+}");
+    }
+
 }