Initial work to support EnumeratorCancellation.

Ix.NET/Source/System.Linq.Async/System/Linq/Operators/Cast.cs

@@ -30,9 +30,15 @@ namespace System.Linq
                 return typedSource;
             }
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TResult> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TResult> Core(CancellationToken cancellationToken)
+#endif
             {
                 await foreach (var obj in source.WithCancellation(cancellationToken).ConfigureAwait(false))
                 {

Ix.NET/Source/System.Linq.Async/System/Linq/Operators/Except.cs

@@ -37,9 +37,15 @@ namespace System.Linq
             if (second == null)
                 throw Error.ArgumentNull(nameof(second));
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TSource> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TSource> Core(CancellationToken cancellationToken)
+#endif
             {
                 var set = new Set<TSource>(comparer);
 

Ix.NET/Source/System.Linq.Async/System/Linq/Operators/GroupJoin.cs

@@ -55,9 +55,15 @@ namespace System.Linq
             if (resultSelector == null)
                 throw Error.ArgumentNull(nameof(resultSelector));
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TResult> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TResult> Core(CancellationToken cancellationToken)
+#endif
             {
                 await using var e = outer.GetConfiguredAsyncEnumerator(cancellationToken, false);
 
@@ -92,9 +98,15 @@ namespace System.Linq
             if (resultSelector == null)
                 throw Error.ArgumentNull(nameof(resultSelector));
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TResult> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TResult> Core(CancellationToken cancellationToken)
+#endif
             {
                 await using var e = outer.GetConfiguredAsyncEnumerator(cancellationToken, false);
 
@@ -130,9 +142,15 @@ namespace System.Linq
             if (resultSelector == null)
                 throw Error.ArgumentNull(nameof(resultSelector));
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TResult> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TResult> Core(CancellationToken cancellationToken)
+#endif
             {
                 await using var e = outer.GetConfiguredAsyncEnumerator(cancellationToken, false);
 

Ix.NET/Source/System.Linq.Async/System/Linq/Operators/Intersect.cs

@@ -37,9 +37,15 @@ namespace System.Linq
             if (second == null)
                 throw Error.ArgumentNull(nameof(second));
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TSource> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TSource> Core(CancellationToken cancellationToken)
+#endif
             {
                 var set = new Set<TSource>(comparer);
 

Ix.NET/Source/System.Linq.Async/System/Linq/Operators/Join.cs

@@ -55,9 +55,15 @@ namespace System.Linq
             if (resultSelector == null)
                 throw Error.ArgumentNull(nameof(resultSelector));
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TResult> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TResult> Core(CancellationToken cancellationToken)
+#endif
             {
                 await using var e = outer.GetConfiguredAsyncEnumerator(cancellationToken, false);
 
@@ -108,9 +114,15 @@ namespace System.Linq
             if (resultSelector == null)
                 throw Error.ArgumentNull(nameof(resultSelector));
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TResult> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TResult> Core(CancellationToken cancellationToken)
+#endif
             {
                 await using var e = outer.GetConfiguredAsyncEnumerator(cancellationToken, false);
 
@@ -162,9 +174,15 @@ namespace System.Linq
             if (resultSelector == null)
                 throw Error.ArgumentNull(nameof(resultSelector));
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TResult> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TResult> Core(CancellationToken cancellationToken)
+#endif
             {
                 await using var e = outer.GetConfiguredAsyncEnumerator(cancellationToken, false);
 

Ix.NET/Source/System.Linq.Async/System/Linq/Operators/OfType.cs

@@ -30,9 +30,15 @@ namespace System.Linq
             if (source == null)
                 throw Error.ArgumentNull(nameof(source));
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TResult> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TResult> Core(CancellationToken cancellationToken)
+#endif
             {
                 await foreach (var obj in source.WithCancellation(cancellationToken).ConfigureAwait(false))
                 {

Ix.NET/Source/System.Linq.Async/System/Linq/Operators/Select.cs

@@ -50,9 +50,15 @@ namespace System.Linq
             if (selector == null)
                 throw Error.ArgumentNull(nameof(selector));
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TResult> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TResult> Core(CancellationToken cancellationToken)
+#endif
             {
                 var index = -1;
 
@@ -107,9 +113,15 @@ namespace System.Linq
             if (selector == null)
                 throw Error.ArgumentNull(nameof(selector));
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TResult> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TResult> Core(CancellationToken cancellationToken)
+#endif
             {
                 var index = -1;
 
@@ -133,9 +145,15 @@ namespace System.Linq
             if (selector == null)
                 throw Error.ArgumentNull(nameof(selector));
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TResult> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TResult> Core(CancellationToken cancellationToken)
+#endif
             {
                 var index = -1;
 

Ix.NET/Source/System.Linq.Async/System/Linq/Operators/SelectMany.cs

@@ -69,9 +69,15 @@ namespace System.Linq
             if (selector == null)
                 throw Error.ArgumentNull(nameof(selector));
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TResult> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TResult> Core(CancellationToken cancellationToken)
+#endif
             {
                 var index = -1;
 
@@ -99,9 +105,15 @@ namespace System.Linq
             if (selector == null)
                 throw Error.ArgumentNull(nameof(selector));
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TResult> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TResult> Core(CancellationToken cancellationToken)
+#endif
             {
                 var index = -1;
 
@@ -130,9 +142,15 @@ namespace System.Linq
             if (selector == null)
                 throw Error.ArgumentNull(nameof(selector));
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TResult> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TResult> Core(CancellationToken cancellationToken)
+#endif
             {
                 var index = -1;
 
@@ -174,9 +192,15 @@ namespace System.Linq
             if (resultSelector == null)
                 throw Error.ArgumentNull(nameof(resultSelector));
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TResult> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TResult> Core(CancellationToken cancellationToken)
+#endif
             {
                 await foreach (var element in source.WithCancellation(cancellationToken).ConfigureAwait(false))
                 {
@@ -199,9 +223,15 @@ namespace System.Linq
             if (resultSelector == null)
                 throw Error.ArgumentNull(nameof(resultSelector));
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TResult> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TResult> Core(CancellationToken cancellationToken)
+#endif
             {
                 await foreach (var element in source.WithCancellation(cancellationToken).ConfigureAwait(false))
                 {
@@ -225,9 +255,15 @@ namespace System.Linq
             if (resultSelector == null)
                 throw Error.ArgumentNull(nameof(resultSelector));
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TResult> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TResult> Core(CancellationToken cancellationToken)
+#endif
             {
                 await foreach (var element in source.WithCancellation(cancellationToken).ConfigureAwait(false))
                 {
@@ -262,9 +298,15 @@ namespace System.Linq
             if (resultSelector == null)
                 throw Error.ArgumentNull(nameof(resultSelector));
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TResult> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TResult> Core(CancellationToken cancellationToken)
+#endif
             {
                 var index = -1;
 
@@ -294,9 +336,15 @@ namespace System.Linq
             if (resultSelector == null)
                 throw Error.ArgumentNull(nameof(resultSelector));
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TResult> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TResult> Core(CancellationToken cancellationToken)
+#endif
             {
                 var index = -1;
 
@@ -327,9 +375,15 @@ namespace System.Linq
             if (resultSelector == null)
                 throw Error.ArgumentNull(nameof(resultSelector));
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TResult> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TResult> Core(CancellationToken cancellationToken)
+#endif
             {
                 var index = -1;
 

Ix.NET/Source/System.Linq.Async/System/Linq/Operators/SkipLast.cs

@@ -41,9 +41,15 @@ namespace System.Linq
                 count = 0;
             }
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TSource> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TSource> Core(CancellationToken cancellationToken)
+#endif
             {
                 var queue = new Queue<TSource>();
 

Ix.NET/Source/System.Linq.Async/System/Linq/Operators/SkipWhile.cs

@@ -25,9 +25,15 @@ namespace System.Linq
             if (predicate == null)
                 throw Error.ArgumentNull(nameof(predicate));
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TSource> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TSource> Core(CancellationToken cancellationToken)
+#endif
             {
                 await using var e = source.GetConfiguredAsyncEnumerator(cancellationToken, false);
 
@@ -66,9 +72,15 @@ namespace System.Linq
             if (predicate == null)
                 throw Error.ArgumentNull(nameof(predicate));
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TSource> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TSource> Core(CancellationToken cancellationToken)
+#endif
             {
                 await using var e = source.GetConfiguredAsyncEnumerator(cancellationToken, false);
                 var index = -1;
@@ -104,9 +116,15 @@ namespace System.Linq
             if (predicate == null)
                 throw Error.ArgumentNull(nameof(predicate));
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TSource> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TSource> Core(CancellationToken cancellationToken)
+#endif
             {
                 await using var e = source.GetConfiguredAsyncEnumerator(cancellationToken, false);
 
@@ -137,9 +155,15 @@ namespace System.Linq
             if (predicate == null)
                 throw Error.ArgumentNull(nameof(predicate));
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TSource> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TSource> Core(CancellationToken cancellationToken)
+#endif
             {
                 await using var e = source.GetConfiguredAsyncEnumerator(cancellationToken, false);
 
@@ -170,9 +194,15 @@ namespace System.Linq
             if (predicate == null)
                 throw Error.ArgumentNull(nameof(predicate));
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TSource> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TSource> Core(CancellationToken cancellationToken)
+#endif
             {
                 await using var e = source.GetConfiguredAsyncEnumerator(cancellationToken, false);
                 var index = -1;
@@ -209,9 +239,15 @@ namespace System.Linq
             if (predicate == null)
                 throw Error.ArgumentNull(nameof(predicate));
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TSource> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TSource> Core(CancellationToken cancellationToken)
+#endif
             {
                 await using var e = source.GetConfiguredAsyncEnumerator(cancellationToken, false);
                 var index = -1;

Ix.NET/Source/System.Linq.Async/System/Linq/Operators/TakeLast.cs

@@ -34,9 +34,15 @@ namespace System.Linq
                 return Empty<TSource>();
             }
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TSource> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TSource> Core(CancellationToken cancellationToken)
+#endif
             {
                 Queue<TSource> queue;
 

Ix.NET/Source/System.Linq.Async/System/Linq/Operators/TakeWhile.cs

@@ -25,9 +25,15 @@ namespace System.Linq
             if (predicate == null)
                 throw Error.ArgumentNull(nameof(predicate));
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TSource> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TSource> Core(CancellationToken cancellationToken)
+#endif
             {
                 await foreach (var element in source.WithCancellation(cancellationToken).ConfigureAwait(false))
                 {
@@ -57,9 +63,15 @@ namespace System.Linq
             if (predicate == null)
                 throw Error.ArgumentNull(nameof(predicate));
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TSource> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TSource> Core(CancellationToken cancellationToken)
+#endif
             {
                 var index = -1;
 
@@ -87,9 +99,15 @@ namespace System.Linq
             if (predicate == null)
                 throw Error.ArgumentNull(nameof(predicate));
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TSource> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TSource> Core(CancellationToken cancellationToken)
+#endif
             {
                 await foreach (var element in source.WithCancellation(cancellationToken).ConfigureAwait(false))
                 {
@@ -111,9 +129,15 @@ namespace System.Linq
             if (predicate == null)
                 throw Error.ArgumentNull(nameof(predicate));
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TSource> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TSource> Core(CancellationToken cancellationToken)
+#endif
             {
                 await foreach (var element in source.WithCancellation(cancellationToken).ConfigureAwait(false))
                 {
@@ -135,9 +159,15 @@ namespace System.Linq
             if (predicate == null)
                 throw Error.ArgumentNull(nameof(predicate));
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TSource> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TSource> Core(CancellationToken cancellationToken)
+#endif
             {
                 var index = -1;
 
@@ -166,9 +196,15 @@ namespace System.Linq
             if (predicate == null)
                 throw Error.ArgumentNull(nameof(predicate));
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TSource> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TSource> Core(CancellationToken cancellationToken)
+#endif
             {
                 var index = -1;
 

Ix.NET/Source/System.Linq.Async/System/Linq/Operators/Where.cs

@@ -49,9 +49,15 @@ namespace System.Linq
             if (predicate == null)
                 throw Error.ArgumentNull(nameof(predicate));
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TSource> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TSource> Core(CancellationToken cancellationToken)
+#endif
             {
                 var index = -1;
 
@@ -111,9 +117,15 @@ namespace System.Linq
             if (predicate == null)
                 throw Error.ArgumentNull(nameof(predicate));
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TSource> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TSource> Core(CancellationToken cancellationToken)
+#endif
             {
                 var index = -1;
 
@@ -140,9 +152,15 @@ namespace System.Linq
             if (predicate == null)
                 throw Error.ArgumentNull(nameof(predicate));
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TSource> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TSource> Core(CancellationToken cancellationToken)
+#endif
             {
                 var index = -1;
 

Ix.NET/Source/System.Linq.Async/System/Linq/Operators/Zip.cs

@@ -18,9 +18,15 @@ namespace System.Linq
             if (second == null)
                 throw Error.ArgumentNull(nameof(second));
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<(TFirst, TSecond)> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<(TFirst, TSecond)> Core(CancellationToken cancellationToken)
+#endif
             {
                 await using var e1 = first.GetConfiguredAsyncEnumerator(cancellationToken, false);
                 await using var e2 = second.GetConfiguredAsyncEnumerator(cancellationToken, false);
@@ -53,9 +59,15 @@ namespace System.Linq
             if (selector == null)
                 throw Error.ArgumentNull(nameof(selector));
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TResult> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TResult> Core(CancellationToken cancellationToken)
+#endif
             {
                 await using var e1 = first.GetConfiguredAsyncEnumerator(cancellationToken, false);
                 await using var e2 = second.GetConfiguredAsyncEnumerator(cancellationToken, false);
@@ -76,9 +88,15 @@ namespace System.Linq
             if (selector == null)
                 throw Error.ArgumentNull(nameof(selector));
 
+#if HAS_ASYNC_ENUMERABLE_CANCELLATION
+            return Core();
+
+            async IAsyncEnumerable<TResult> Core([System.Runtime.CompilerServices.EnumeratorCancellation]CancellationToken cancellationToken = default)
+#else
             return Create(Core);
 
             async IAsyncEnumerator<TResult> Core(CancellationToken cancellationToken)
+#endif
             {
                 await using var e1 = first.GetConfiguredAsyncEnumerator(cancellationToken, false);
                 await using var e2 = second.GetConfiguredAsyncEnumerator(cancellationToken, false);

Ix.NET/Source/System.Linq.Async/System/Runtime/CompilerServices/EnumeratorCancellationAttribute.cs

@@ -0,0 +1,15 @@
+#if !BCL_HAS_ENUMERATOR_CANCELLATION
+
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+namespace System.Runtime.CompilerServices
+{
+    [AttributeUsage(AttributeTargets.Parameter, Inherited = false)]
+    public sealed class EnumeratorCancellationAttribute : Attribute
+    {
+    }
+}
+
+#endif