test to catch onlyIfCheap true

Ix.NET/Source/Tests/AsyncTests.Single.cs

@@ -1513,6 +1513,16 @@ namespace Tests
             await SequenceIdentity(ys);
         }
 
+        [Fact]
+        public async Task Reverse9()
+        {
+            var xs = new[] { 1, 2, 3 }.ToAsyncEnumerable();
+            var ys = xs.Reverse().Prepend(4); // to trigger onlyIfCheap
+
+            Assert.Equal(new[] { 4, 3, 2, 1 }, await ys.ToArray());
+        }
+
+
         [Fact]
         public void OrderBy_Null()
         {