Fix #3384: add special case for null-literal conversions in optional argument handling.

Author: siegfriedpammer

ICSharpCode.Decompiler.Tests/TestCases/Pretty/OptionalArguments.cs

@@ -32,6 +32,28 @@ public enum MyEnum
 			B
 		}
 
+		internal class OptionalArgumentTest
+		{
+			private static void Test()
+			{
+				Test2();
+				Test3();
+				Test4();
+			}
+
+			private static void Test2(int a = 0)
+			{
+			}
+
+			private static void Test3(int a = 0, int? b = null)
+			{
+			}
+
+			private static void Test4(int? b = null, int a = 0)
+			{
+			}
+		}
+
 		public OptionalArguments(string name, int a = 5)
 		{
 

ICSharpCode.Decompiler/CSharp/CallBuilder.cs

@@ -1043,7 +1043,10 @@ bool CheckArgument(out int len, out IType t)
 
 		bool IsOptionalArgument(IParameter parameter, TranslatedExpression arg)
 		{
-			if (!parameter.IsOptional || !arg.ResolveResult.IsCompileTimeConstant)
+			if (!parameter.IsOptional)
+				return false;
+
+			if (!arg.ResolveResult.IsCompileTimeConstant && arg.ResolveResult is not ConversionResolveResult { Conversion.IsNullLiteralConversion: true })
 				return false;
 			if (parameter.GetAttributes().Any(a => a.AttributeType.IsKnownType(KnownAttribute.CallerMemberName)
 				|| a.AttributeType.IsKnownType(KnownAttribute.CallerFilePath)