Fix LINQ Last() in ConcatNIterator to also check base case (Concat2Iterator) (#108486)

* Fix LINQ Last() in ConcatNIterator to also check base case (Concat2Iterator)

* Add a few more test cases of empty concats

* Simplify solution as suggested in PR Review

Author: andrewjsaid

src/libraries/System.Linq/src/System/Linq/Concat.SpeedOpt.cs

@@ -326,19 +326,21 @@ private TSource[] PreallocatingToArray()
 
             public override TSource? TryGetLast(out bool found)
             {
-                ConcatNIterator<TSource>? node = this;
+                ConcatNIterator<TSource>? node, previousN = this;
+                TSource? result;
                 do
                 {
-                    TSource? result = node._head.TryGetLast(out found);
+                    node = previousN;
+                    result = node._head.TryGetLast(out found);
                     if (found)
                     {
                         return result;
                     }
                 }
-                while ((node = node!.PreviousN) is not null);
+                while ((previousN = node.PreviousN) is not null);
 
-                found = false;
-                return default;
+                Debug.Assert(node._tail is Concat2Iterator<TSource>);
+                return node._tail.TryGetLast(out found);
             }
         }
 

src/libraries/System.Linq/tests/ConcatTests.cs

@@ -93,6 +93,7 @@ public void VerifyEquals(IEnumerable<int> expected, IEnumerable<int> actual)
         [MemberData(nameof(ConcatWithSelfData))]
         [MemberData(nameof(ChainedCollectionConcatData))]
         [MemberData(nameof(AppendedPrependedConcatAlternationsData))]
+        [MemberData(nameof(ConcatWithEmptyEnumerableData))]
         public void First_Last_ElementAt(IEnumerable<int> _, IEnumerable<int> actual)
         {
             int count = actual.Count();
@@ -232,6 +233,32 @@ public static IEnumerable<object[]> AppendedPrependedConcatAlternationsData()
             }
         }
 
+        public static IEnumerable<object[]> ConcatWithEmptyEnumerableData()
+        {
+            List<int> baseList = [0, 1, 2, 3, 4];
+
+            yield return new object[]
+            {
+                Enumerable.Range(0, 5),
+                Enumerable.Concat(Enumerable.Concat(new List<int>(), new List<int>()), baseList)
+            };
+            yield return new object[]
+            {
+                Enumerable.Range(0, 5),
+                Enumerable.Concat(new List<int>(), baseList)
+            };
+            yield return new object[]
+            {
+                Enumerable.Range(0, 5),
+                Enumerable.Concat(Enumerable.Concat(baseList, new List<int>()), new List<int>())
+            };
+            yield return new object[]
+            {
+                Enumerable.Range(0, 5),
+                Enumerable.Concat(baseList, new List<int>())
+            };
+        }
+
         private static IEnumerable<object[]> GenerateSourcesData(
             Func<IEnumerable<int>, IEnumerable<int>> outerTransform = null,
             Func<IEnumerable<int>, IEnumerable<int>> innerTransform = null)