Add unit test for lamba function

Author: ds5678

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

@@ -308,11 +308,15 @@ public static void Issue2920p(in int x = 3)
 		{
 		}
 #endif
-		public static void Issue3469([Optional][DefaultParameterValue(0)] int i, [Optional] DateTime d)
+		public static void Issue3469a([Optional][DefaultParameterValue(0)] int i, [Optional] DateTime d)
 		{
 		}
-
 #if CS120
+		public static Action<int, DateTime> Issue3469b()
+		{
+			return ([Optional][DefaultParameterValue(0)] int i, [Optional] DateTime d) => {
+			};
+		}
 		public static D LambdaWithOptionalParameter()
 		{
 			return (int x = 10) => x;

ICSharpCode.Decompiler/TypeSystem/TypeSystemExtensions.cs

@@ -629,7 +629,7 @@ public static bool IsDefaultValueAssignmentAllowed(this IParameter parameter)
 				return false;
 
 			if (parameter.Owner == null)
-				return true;
+				return true; // Shouldn't happen, but we need to check for it.
 
 			for (int i = parameter.Owner.Parameters.Count - 1; i >= 0; i--)
 			{