Review feedback

Allow out-of-bounds index on empty array if count is zero.

src/libraries/System.Collections.Concurrent/src/System/Collections/Concurrent/ConcurrentStack.cs

@@ -317,10 +317,6 @@ public void PushRange(T[] items)
                 throw new ArgumentNullException(nameof(items));
             }
 
-            // No op if the length is zero
-            if (items.Length == 0)
-                return;
-
             PushRange(items, 0, items.Length);
         }
 
@@ -414,7 +410,7 @@ private static void ValidatePushPopRangeInput(T[] items, int startIndex, int cou
                 throw new ArgumentOutOfRangeException(nameof(count), SR.ConcurrentStack_PushPopRange_CountOutOfRange);
             }
             int length = items.Length;
-            if (startIndex >= length || startIndex < 0)
+            if (startIndex > length || startIndex < 0)
             {
                 throw new ArgumentOutOfRangeException(nameof(startIndex), SR.ConcurrentStack_PushPopRange_StartOutOfRange);
             }

src/libraries/System.Collections.Concurrent/tests/ConcurrentStackTests.cs

@@ -64,7 +64,17 @@ public void PushRange_NoItems_NothingAdded()
         }
 
         [Fact]
-        public void PushRange_NoItems_NothingAdded_NoRangeSpecified()
+        public void PushRange_NoItems_NothingAdded_EmptyArrayWithRangeSpecified()
+        {
+            var s = new ConcurrentStack<int>();
+            Assert.True(s.IsEmpty);
+
+            s.PushRange(new int[0], 0, 0);
+            Assert.True(s.IsEmpty);
+        }
+
+        [Fact]
+        public void PushRange_NoItems_NothingAdded_EmptyArrayNoRangeSpecified()
         {
             var s = new ConcurrentStack<int>();
             Assert.True(s.IsEmpty);
@@ -141,11 +151,10 @@ public void PushRange_InvalidArguments_Throws()
             var stack = new ConcurrentStack<int>();
             Assert.Throws<ArgumentNullException>(() => stack.PushRange(null));
             Assert.Throws<ArgumentNullException>(() => stack.PushRange(null, 0, 0));
-            Assert.Throws<ArgumentOutOfRangeException>(() => stack.PushRange(new int[0], 0, 0));
-            Assert.Throws<ArgumentOutOfRangeException>(() => stack.PushRange(new int[0], 0, 1));
             Assert.Throws<ArgumentOutOfRangeException>(() => stack.PushRange(new int[1], 0, -1));
             Assert.Throws<ArgumentOutOfRangeException>(() => stack.PushRange(new int[1], -1, 1));
             Assert.Throws<ArgumentOutOfRangeException>(() => stack.PushRange(new int[1], 2, 1));
+            AssertExtensions.Throws<ArgumentException>(null, () => stack.PushRange(new int[0], 0, 1));
             AssertExtensions.Throws<ArgumentException>(null, () => stack.PushRange(new int[1], 0, 10));
         }