Update SA1204 for default interface members

Author: sharwell

StyleCop.Analyzers/StyleCop.Analyzers.Test.CSharp8/OrderingRules/SA1204CSharp8UnitTests.cs

@@ -3,9 +3,46 @@
 
 namespace StyleCop.Analyzers.Test.CSharp8.OrderingRules
 {
+    using System.Threading;
+    using System.Threading.Tasks;
+    using Microsoft.CodeAnalysis.Testing;
     using StyleCop.Analyzers.Test.CSharp7.OrderingRules;
+    using Xunit;
+    using static StyleCop.Analyzers.Test.Verifiers.StyleCopCodeFixVerifier<
+        StyleCop.Analyzers.OrderingRules.SA1204StaticElementsMustAppearBeforeInstanceElements,
+        StyleCop.Analyzers.OrderingRules.ElementOrderCodeFixProvider>;
 
     public partial class SA1204CSharp8UnitTests : SA1204CSharp7UnitTests
     {
+        [Fact]
+        [WorkItem(3002, "https://github.com/DotNetAnalyzers/StyleCopAnalyzers/issues/3002")]
+        public async Task TestStaticMemberAfterDefaultInterfaceMethodAsync()
+        {
+            var testCode = @"public interface ITest
+{
+    void InstanceMethod()
+    {
+    }
+
+    static void [|StaticMethod|]()
+    {
+    }
+}
+";
+
+            var fixedCode = @"public interface ITest
+{
+    static void StaticMethod()
+    {
+    }
+
+    void InstanceMethod()
+    {
+    }
+}
+";
+
+            await VerifyCSharpFixAsync(testCode, DiagnosticResult.EmptyDiagnosticResults, fixedCode, CancellationToken.None).ConfigureAwait(false);
+        }
     }
 }

StyleCop.Analyzers/StyleCop.Analyzers/OrderingRules/SA1204StaticElementsMustAppearBeforeInstanceElements.cs

@@ -41,7 +41,7 @@ internal class SA1204StaticElementsMustAppearBeforeInstanceElements : Diagnostic
             new DiagnosticDescriptor(DiagnosticId, Title, MessageFormat, AnalyzerCategory.OrderingRules, DiagnosticSeverity.Warning, AnalyzerConstants.EnabledByDefault, Description, HelpLink);
 
         private static readonly ImmutableArray<SyntaxKind> TypeDeclarationKinds =
-            ImmutableArray.Create(SyntaxKind.ClassDeclaration, SyntaxKind.StructDeclaration);
+            ImmutableArray.Create(SyntaxKind.ClassDeclaration, SyntaxKind.StructDeclaration, SyntaxKind.InterfaceDeclaration);
 
         private static readonly Action<SyntaxNodeAnalysisContext, StyleCopSettings> CompilationUnitAction = HandleCompilationUnit;
         private static readonly Action<SyntaxNodeAnalysisContext, StyleCopSettings> BaseNamespaceDeclarationAction = HandleBaseNamespaceDeclaration;

documentation/SA1204.md

@@ -23,6 +23,28 @@ A static element is positioned beneath an instance element of the same type.
 
 A violation of this rule occurs when a static element is positioned beneath an instance element of the same type. All static elements should be placed above all instance elements of the same type to make it easier to see the interface exposed from the instance and static version of the class.
 
+This rule applies to classes, structs, and interfaces. Static interface members (introduced with C# 8 default interface members) must appear before instance members, including members that provide default implementations.
+
+For example, the following interface declaration violates this rule because the static member appears after an instance member with a default implementation:
+
+```csharp
+public interface IExample
+{
+    void Instance() { }
+    static void Helper() { } // SA1204
+}
+```
+
+A compliant ordering places static interface members before instance members:
+
+```csharp
+public interface IExample
+{
+    static void Helper() { }
+    void Instance() { }
+}
+```
+
 ## How to fix violations
 
 To fix an instance of this violation, place all static elements above all instance elements of the same type.