Add SA1106 tests for extension foreach

Author: sharwell

StyleCop.Analyzers/StyleCop.Analyzers.Test.CSharp9/ReadabilityRules/SA1106CSharp9UnitTests.cs

@@ -1,8 +1,6 @@
 // Copyright (c) Tunnel Vision Laboratories, LLC. All Rights Reserved.
 // Licensed under the MIT License. See LICENSE in the project root for license information.
 
-#nullable disable
-
 namespace StyleCop.Analyzers.Test.CSharp9.ReadabilityRules
 {
     using System.Threading;
@@ -28,5 +26,71 @@ public async Task TestNoDiagnosticForEmptyRecordDeclarationAsync()
                 TestCode = testCode,
             }.RunAsync(CancellationToken.None).ConfigureAwait(false);
         }
+
+        [Fact]
+        [WorkItem(3976, "https://github.com/DotNetAnalyzers/StyleCopAnalyzers/issues/3976")]
+        public async Task TestForeachWithExtensionEnumeratorAsync()
+        {
+            var testCode = @"
+using System.Collections.Generic;
+
+public class TestClass
+{
+    public void TestMethod()
+    {
+        foreach (var value in new ExtensionEnumerable())
+            [|;|]
+    }
+}
+
+public class ExtensionEnumerable
+{
+}
+
+public struct ExtensionEnumerator
+{
+    public int Current => 0;
+
+    public bool MoveNext() => false;
+}
+
+public static class ExtensionEnumerableExtensions
+{
+    public static ExtensionEnumerator GetEnumerator(this ExtensionEnumerable enumerable) => new();
+}
+";
+
+            var fixedCode = @"
+using System.Collections.Generic;
+
+public class TestClass
+{
+    public void TestMethod()
+    {
+        foreach (var value in new ExtensionEnumerable())
+        {
+        }
+    }
+}
+
+public class ExtensionEnumerable
+{
+}
+
+public struct ExtensionEnumerator
+{
+    public int Current => 0;
+
+    public bool MoveNext() => false;
+}
+
+public static class ExtensionEnumerableExtensions
+{
+    public static ExtensionEnumerator GetEnumerator(this ExtensionEnumerable enumerable) => new();
+}
+";
+
+            await VerifyCSharpFixAsync(testCode, DiagnosticResult.EmptyDiagnosticResults, fixedCode, CancellationToken.None).ConfigureAwait(false);
+        }
     }
 }