Add SA1312 tests for extension foreach

Author: sharwell

StyleCop.Analyzers/StyleCop.Analyzers.Test.CSharp9/NamingRules/SA1312CSharp9UnitTests.cs

@@ -3,9 +3,85 @@
 
 namespace StyleCop.Analyzers.Test.CSharp9.NamingRules
 {
+    using System.Threading;
+    using System.Threading.Tasks;
+    using Microsoft.CodeAnalysis.Testing;
     using StyleCop.Analyzers.Test.CSharp8.NamingRules;
+    using StyleCop.Analyzers.Test.Helpers;
+    using Xunit;
+    using static StyleCop.Analyzers.Test.Verifiers.StyleCopCodeFixVerifier<
+        StyleCop.Analyzers.NamingRules.SA1312VariableNamesMustBeginWithLowerCaseLetter,
+        StyleCop.Analyzers.NamingRules.RenameToLowerCaseCodeFixProvider>;
 
     public partial class SA1312CSharp9UnitTests : SA1312CSharp8UnitTests
     {
+        [Fact]
+        [WorkItem(3976, "https://github.com/DotNetAnalyzers/StyleCopAnalyzers/issues/3976")]
+        public async Task TestForeachVariableWithExtensionEnumeratorAsync()
+        {
+            var testCode = @"
+using System;
+
+public class TestClass
+{
+    public void TestMethod()
+    {
+        foreach (var [|Item|] in new ExtensionEnumerable())
+        {
+            Console.WriteLine(Item);
+        }
+    }
+}
+
+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;
+
+public class TestClass
+{
+    public void TestMethod()
+    {
+        foreach (var item in new ExtensionEnumerable())
+        {
+            Console.WriteLine(item);
+        }
+    }
+}
+
+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);
+        }
     }
 }