fix: OfType now accepts a collection of object?

fixes #1525

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

@@ -1,6 +1,6 @@
 // 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. 
+// See the LICENSE file in the project root for more information.
 
 using System;
 using System.Linq;
@@ -40,5 +40,17 @@ namespace Tests
             await HasNextAsync(e, "foo");
             await NoNextAsync(e);
         }
+
+        [Fact]
+        public async Task OfType_NotNullObject()
+        {
+            var xs = new object?[] { 42, null, "foo", null }.ToAsyncEnumerable();
+            var ys = xs.OfType<object>();
+
+            var e = ys.GetAsyncEnumerator();
+            await HasNextAsync(e, 42);
+            await HasNextAsync(e, "foo");
+            await NoNextAsync(e);
+        }
     }
 }

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

@@ -1,6 +1,6 @@
 // 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. 
+// See the LICENSE file in the project root for more information.
 
 using System.Collections.Generic;
 using System.Threading;
@@ -25,14 +25,14 @@ namespace System.Linq
         /// <param name="source">The async-enumerable sequence that contains the elements to be filtered.</param>
         /// <returns>An async-enumerable sequence that contains elements from the input sequence of type TResult.</returns>
         /// <exception cref="ArgumentNullException"><paramref name="source"/> is null.</exception>
-        public static IAsyncEnumerable<TResult> OfType<TResult>(this IAsyncEnumerable<object> source)
+        public static IAsyncEnumerable<TResult> OfType<TResult>(this IAsyncEnumerable<object?> source)
         {
             if (source == null)
                 throw Error.ArgumentNull(nameof(source));
 
             return Core(source);
 
-            static async IAsyncEnumerable<TResult> Core(IAsyncEnumerable<object> source, [System.Runtime.CompilerServices.EnumeratorCancellation] CancellationToken cancellationToken = default)
+            static async IAsyncEnumerable<TResult> Core(IAsyncEnumerable<object?> source, [System.Runtime.CompilerServices.EnumeratorCancellation] CancellationToken cancellationToken = default)
             {
                 await foreach (var obj in source.WithCancellation(cancellationToken).ConfigureAwait(false))
                 {