Fix LINQ handling of iterator.Take(...).Last(...)

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

@@ -426,9 +426,12 @@ public override Iterator<TSource> Take(int count)
             {
                 if (_source is Iterator<TSource> iterator &&
                     iterator.GetCount(onlyIfCheap: true) is int count &&
-                    count >= _minIndexInclusive)
+                    count > _minIndexInclusive)
                 {
-                    return !HasLimit ?
+                    // If there's no upper bound, or if there are fewer items in the list
+                    // than the upper bound allows, just return the last element of the list.
+                    // Otherwise, get the element at the upper bound.
+                    return (uint)count <= (uint)_maxIndexInclusive ?
                         iterator.TryGetLast(out found) :
                         iterator.TryGetElementAt(_maxIndexInclusive, out found);
                 }

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

@@ -11,10 +11,10 @@ public class AggregateByTests : EnumerableTests
         [Fact]
         public void Empty()
         {
-            Assert.All(IdentityTransforms<int>(), transform =>
+            Assert.All(CreateSources<int>([]), source =>
             {
-                Assert.Equal([], transform([]).AggregateBy(i => i, i => i, (a, i) => a + i));
-                Assert.Equal([], transform([]).AggregateBy(i => i, 0, (a, i) => a + i));
+                Assert.Equal([], source.AggregateBy(i => i, i => i, (a, i) => a + i));
+                Assert.Equal([], source.AggregateBy(i => i, 0, (a, i) => a + i));
             });
         }
 

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

@@ -42,10 +42,8 @@ public void ChunkSourceLazily()
         [InlineData(new[] {9999, 0, 888, -1, 66, -777, 1, 2, -12345})]
         public void ChunkSourceRepeatCalls(int[] array)
         {
-            Assert.All(IdentityTransforms<int>(), t =>
+            Assert.All(CreateSources(array), source =>
             {
-                IEnumerable<int> source = t(array);
-
                 Assert.Equal(source.Chunk(3), source.Chunk(3));
             });
         }
@@ -54,10 +52,8 @@ public void ChunkSourceRepeatCalls(int[] array)
         [InlineData(new[] {9999, 0, 888, -1, 66, -777, 1, 2, -12345})]
         public void ChunkSourceEvenly(int[] array)
         {
-            Assert.All(IdentityTransforms<int>(), t =>
+            Assert.All(CreateSources(array), source =>
             {
-                IEnumerable<int> source = t(array);
-
                 using IEnumerator<int[]> chunks = source.Chunk(3).GetEnumerator();
                 chunks.MoveNext();
                 Assert.Equal(new[] { 9999, 0, 888 }, chunks.Current);
@@ -73,10 +69,8 @@ public void ChunkSourceEvenly(int[] array)
         [InlineData(new[] {9999, 0, 888, -1, 66, -777, 1, 2})]
         public void ChunkSourceUnevenly(int[] array)
         {
-            Assert.All(IdentityTransforms<int>(), t =>
+            Assert.All(CreateSources(array), source =>
             {
-                IEnumerable<int> source = t(array);
-
                 using IEnumerator<int[]> chunks = source.Chunk(3).GetEnumerator();
                 chunks.MoveNext();
                 Assert.Equal(new[] { 9999, 0, 888 }, chunks.Current);
@@ -92,10 +86,8 @@ public void ChunkSourceUnevenly(int[] array)
         [InlineData(new[] {9999, 0})]
         public void ChunkSourceSmallerThanMaxSize(int[] array)
         {
-            Assert.All(IdentityTransforms<int>(), t =>
+            Assert.All(CreateSources(array), source =>
             {
-                IEnumerable<int> source = t(array);
-
                 using IEnumerator<int[]> chunks = source.Chunk(3).GetEnumerator();
                 chunks.MoveNext();
                 Assert.Equal(new[] { 9999, 0 }, chunks.Current);
@@ -107,10 +99,8 @@ public void ChunkSourceSmallerThanMaxSize(int[] array)
         [InlineData(new int[0])]
         public void EmptySourceYieldsNoChunks(int[] array)
         {
-            Assert.All(IdentityTransforms<int>(), t =>
+            Assert.All(CreateSources(array), source =>
             {
-                IEnumerable<int> source = t(array);
-
                 using IEnumerator<int[]> chunks = source.Chunk(3).GetEnumerator();
                 Assert.False(chunks.MoveNext());
             });

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

@@ -31,8 +31,8 @@ private static void SameResultsWithQueryAndRepeatCallsWorker<T>(IEnumerable<T> f
             first = from item in first select item;
             second = from item in second select item;
 
-            VerifyEqualsWorker(first.Concat(second), first.Concat(second));
-            VerifyEqualsWorker(second.Concat(first), second.Concat(first));
+            Assert.Equal(first.Concat(second), first.Concat(second));
+            Assert.Equal(second.Concat(first), second.Concat(first));
         }
 
         [Theory]
@@ -41,8 +41,8 @@ private static void SameResultsWithQueryAndRepeatCallsWorker<T>(IEnumerable<T> f
         [InlineData(new int[] { 2, 3, 5, 9 }, new int[] { 8, 10 }, new int[] { 2, 3, 5, 9, 8, 10 })] // Neither side is empty
         public void PossiblyEmptyInputs(IEnumerable<int> first, IEnumerable<int> second, IEnumerable<int> expected)
         {
-            VerifyEqualsWorker(expected, first.Concat(second));
-            VerifyEqualsWorker(expected.Skip(first.Count()).Concat(expected.Take(first.Count())), second.Concat(first)); // Swap the inputs around
+            Assert.Equal(expected, first.Concat(second));
+            Assert.Equal(expected.Skip(first.Count()).Concat(expected.Take(first.Count())), second.Concat(first)); // Swap the inputs around
         }
 
         [Fact]
@@ -80,7 +80,7 @@ public void SecondNull()
         public void VerifyEquals(IEnumerable<int> expected, IEnumerable<int> actual)
         {
             // workaround: xUnit type inference doesn't work if the input type is not T (like IEnumerable<T>)
-            VerifyEqualsWorker(expected, actual);
+            Assert.Equal(expected, actual);
         }
 
         [Theory]
@@ -133,23 +133,6 @@ public void First_Last_ElementAt(IEnumerable<int> _, IEnumerable<int> actual)
             }
         }
 
-        private static void VerifyEqualsWorker<T>(IEnumerable<T> expected, IEnumerable<T> actual)
-        {
-            // Returns a list of functions that, when applied to enumerable, should return
-            // another one that has equivalent contents.
-            var identityTransforms = IdentityTransforms<T>();
-
-            // We run the transforms N^2 times, by testing all transforms
-            // of expected against all transforms of actual.
-            foreach (var outTransform in identityTransforms)
-            {
-                foreach (var inTransform in identityTransforms)
-                {
-                    Assert.Equal(outTransform(expected), inTransform(actual));
-                }
-            }
-        }
-
         public static IEnumerable<object[]> ArraySourcesData() => GenerateSourcesData(outerTransform: e => e.ToArray());
 
         public static IEnumerable<object[]> SelectArraySourcesData() => GenerateSourcesData(outerTransform: e => e.Select(i => i).ToArray());
@@ -292,7 +275,7 @@ public void ManyConcats(IEnumerable<IEnumerable<int>> sources)
                 }
 
                 Assert.Equal(sources.Sum(s => s.Count()), concatee.Count());
-                VerifyEqualsWorker(sources.SelectMany(s => s), concatee);
+                Assert.Equal(sources.SelectMany(s => s), concatee);
             }
         }
 

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

@@ -99,9 +99,9 @@ public void RunOnce<T>(int count, IEnumerable<T> enumerable)
 
         private static IEnumerable<object[]> EnumerateCollectionTypesAndCounts<T>(int count, IEnumerable<T> enumerable)
         {
-            foreach (var transform in IdentityTransforms<T>())
+            foreach (IEnumerable<T> source in CreateSources(enumerable))
             {
-                yield return [count, transform(enumerable)];
+                yield return [count, source];
             }
         }
 

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

@@ -5,6 +5,7 @@
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
 using Xunit;
+using Xunit.Sdk;
 
 namespace System.Linq.Tests
 {
@@ -243,6 +244,7 @@ protected static IEnumerable<T> FlipIsCollection<T>(IEnumerable<T> source)
         {
             return source is ICollection<T> ? ForceNotCollection(source) : new List<T>(source);
         }
+
         protected static T[] Repeat<T>(Func<int, T> factory, int count)
         {
             T[] results = new T[count];
@@ -316,26 +318,83 @@ protected static IEnumerable<IEnumerable<T>> CreateSources<T>(IEnumerable<T> sou
             }
         }
 
-        protected static List<Func<IEnumerable<T>, IEnumerable<T>>> IdentityTransforms<T>()
+        protected static IEnumerable<Func<IEnumerable<T>, IEnumerable<T>>> IdentityTransforms<T>()
         {
-            // All of these transforms should take an enumerable and produce
-            // another enumerable with the same contents.
-            return
+            // Various collection types all representing the same source.
+            List<Func<IEnumerable<T>, IEnumerable<T>>> sources =
             [
-                e => e,
-                e => e.ToArray(),
-                e => e.ToList(),
-                e => e.ToList().Take(int.MaxValue),
+                e => e, // original
+                e => e.ToArray(), // T[]
+                e => e.ToList(), // List<T>
+                e => new ReadOnlyCollection<T>(e.ToArray()), // IList<T> that's not List<T>/T[]
+                e => new TestCollection<T>(e.ToArray()), // ICollection<T> that's not IList<T>
+                e => new TestReadOnlyCollection<T>(e.ToArray()), // IReadOnlyCollection<T> that's not ICollection<T>
+                e => ForceNotCollection(e), // IEnumerable<T> with no other interfaces
+            ];
+            if (typeof(T) == typeof(char))
+            {
+                sources.Add(e => (IEnumerable<T>)(object)string.Concat((IEnumerable<char>)(object)e)); // string
+            }
+
+            // Various transforms that all yield the same elements as the source.
+            List<Func<IEnumerable<T>, IEnumerable<T>>> transforms =
+            [
+                // Append
+                e =>
+                {
+                    T[] values = e.ToArray();
+                    return values.Length == 0 ? [] : values[0..^1].Append(values[^1]);
+                },
+
+                // Concat
+                e => e.Concat(ForceNotCollection<T>([])),
+                e => ForceNotCollection<T>([]).Concat(e),
+
+                // Prepend
+                e =>
+                {
+                    T[] values = e.ToArray();
+                    return values.Length == 0 ? [] : values[1..].Prepend(values[0]);
+                },
+
+                // Reverse
+                e => e.Reverse().Reverse(),
+
+                // Select
                 e => e.Select(i => i),
-                e => e.Select(i => i).Take(int.MaxValue),
-                e => e.Select(i => i).Where(i => true),
+
+                // SelectMany
+                e => e.SelectMany<T, T>(i => [i]),
+
+                // Take
+                e => e.Take(int.MaxValue),
+                e => e.TakeLast(int.MaxValue),
+                e => e.TakeWhile(i => true),
+
+                // Skip
+                e => e.SkipWhile(i => false),
+
+                // Where
                 e => e.Where(i => true),
-                e => e.Concat(Array.Empty<T>()),
-                e => e.Concat(ForceNotCollection(Array.Empty<T>())),
-                e => ForceNotCollection(e),
-                e => ForceNotCollection(e).Skip(0),
-                e => new ReadOnlyCollection<T>(e.ToArray())
             ];
+
+            foreach (Func<IEnumerable<T>, IEnumerable<T>> source in sources)
+            {
+                // Yield the source itself.
+                yield return source;
+
+                foreach (Func<IEnumerable<T>, IEnumerable<T>> transform in transforms)
+                {
+                    // Yield a single transform on the source
+                    yield return e => transform(source(e));
+
+                    foreach (Func<IEnumerable<T>, IEnumerable<T>> transform2 in transforms)
+                    {
+                        // Yield a second transform on the first transform on the source.
+                        yield return e => transform2(transform(source(e)));
+                    }
+                }
+            }
         }
 
         protected sealed class DelegateIterator<TSource> : IEnumerable<TSource>, IEnumerator<TSource>

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

@@ -372,31 +372,23 @@ public void ForcedToEnumeratorDoesntEnumerateIndexedResultSel()
             Assert.False(en is not null && en.MoveNext());
         }
 
-        [Theory]
-        [MemberData(nameof(ParameterizedTestsData))]
-        public void ParameterizedTests(IEnumerable<int> source, Func<int, IEnumerable<int>> selector)
+        [Fact]
+        public void ParameterizedTests()
         {
-            Assert.All(CreateSources(source), source =>
+            for (int i = 1; i <= 20; i++)
             {
-                var expected = source.Select(i => selector(i)).Aggregate((l, r) => l.Concat(r));
-                var actual = source.SelectMany(selector);
+                Assert.All(CreateSources(Enumerable.Range(1, i)), source =>
+                {
+                    Func<int, IEnumerable<int>> selector = n => Enumerable.Range(i, n);
 
-                Assert.Equal(expected, actual);
-                Assert.Equal(expected.Count(), actual.Count()); // SelectMany may employ an optimized Count implementation.
-                Assert.Equal(expected.ToArray(), actual.ToArray());
-                Assert.Equal(expected.ToList(), actual.ToList());
-            });
-        }
+                    var expected = source.Select(i => selector(i)).Aggregate((l, r) => l.Concat(r)).ToArray();
+                    var actual = source.SelectMany(selector);
 
-        public static IEnumerable<object[]> ParameterizedTestsData()
-        {
-            foreach (Func<IEnumerable<int>, IEnumerable<int>> transform in IdentityTransforms<int>())
-            {
-                for (int i = 1; i <= 20; i++)
-                {
-                    Func<int, IEnumerable<int>> selector = n => transform(Enumerable.Range(i, n));
-                    yield return [Enumerable.Range(1, i), selector];
-                }
+                    Assert.Equal(expected, actual);
+                    Assert.Equal(expected.Length, actual.Count()); // SelectMany may employ an optimized Count implementation.
+                    Assert.Equal(expected, actual.ToArray());
+                    Assert.Equal(expected, actual.ToList());
+                });
             }
         }