C#: Add data flow tests for implicit index usage.

csharp/ql/test/library-tests/dataflow/collections/CollectionFlow.cs

@@ -13,6 +13,8 @@ public class CollectionFlow
 
     public static void SinkElem<T>(T[] ts) => Sink(ts[0]);
 
+    public static void SinkLastElem<T>(T[] ts) => Sink(ts[^1]);
+
     public static void SinkListElem<T>(IList<T> list) => Sink(list[0]);
 
     public static void SinkDictValue<T>(IDictionary<int, T> dict) => Sink(dict[0]);
@@ -21,6 +23,8 @@ public class CollectionFlow
 
     public static T First<T>(T[] ts) => ts[0];
 
+    public static T Last<T>(T[] ts) => ts[^1];
+
     public static T ListFirst<T>(IList<T> list) => list[0];
 
     public static T DictIndexZero<T>(IDictionary<int, T> dict) => dict[0];
@@ -73,6 +77,15 @@ public class CollectionFlow
         Sink(First(c.As)); // no flow
     }
 
+    public void ArrayInitializerImplicitIndexFlow()
+    {
+        var a = new A();
+        var c = new CollectionFlow() { As = { [^1] = a } };
+        Sink(c.As[^1]); // flow
+        SinkLastElem(c.As); // flow
+        Sink(Last(c.As)); // flow
+    }
+
     public void ArrayAssignmentFlow()
     {
         var a = new A();
@@ -93,6 +106,16 @@ public class CollectionFlow
         Sink(First(@as)); // no flow
     }
 
+    public void ArrayAssignmentImplicitIndexFlow()
+    {
+        var a = new A();
+        var @as = new A[1];
+        @as[^1] = a;
+        Sink(@as[^1]); // flow
+        SinkLastElem(@as); // flow
+        Sink(Last(@as)); // flow
+    }
+
     public void ListAssignmentFlow()
     {
         var a = new A();