Fix #1689: Add support for null propagation on array access: `arr?[i]`

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

@@ -184,7 +184,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 		{
 			Use(GetMyClass()?.Text ?? "Hello");
 		}
-		
+
 		public void CallOnValueTypeField()
 		{
 			Use(GetMyClass()?.IntVal.ToString());
@@ -265,6 +265,12 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			return (obj as ICollection)?.Count + (obj as ICollection<int>)?.Count;
 		}
 
+		private static void Issue1689(List<byte[]> setsOfNumbers)
+		{
+			Console.WriteLine(setsOfNumbers?[0]?[1].ToString() == "2");
+			Console.WriteLine(setsOfNumbers?[1]?[1].ToString() == null);
+		}
+	
 		private static dynamic DynamicNullProp(dynamic a)
 		{
 			return a?.b.c(1)?.d[10];

ICSharpCode.Decompiler/IL/Transforms/NullPropagationTransform.cs

@@ -18,6 +18,7 @@
 
 using System;
 using System.Diagnostics;
+using System.Linq;
 using ICSharpCode.Decompiler.TypeSystem;
 
 namespace ICSharpCode.Decompiler.IL.Transforms
@@ -211,6 +212,11 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 						return false;
 				} else if (inst is LdLen ldLen) {
 					inst = ldLen.Array;
+				} else if (inst is LdElema ldElema) {
+					inst = ldElema.Array;
+					// ensure the access chain does not contain any 'nullable.unwrap' that aren't directly part of the chain
+					if (ldElema.Indices.Any(i => i.HasFlag(InstructionFlags.MayUnwrapNull)))
+						return false;
 				} else if (inst is NullableUnwrap unwrap) {
 					inst = unwrap.Argument;
 					if (unwrap.RefInput && inst is AddressOf addressOf) {