Update SA1205 for partial methods

Author: sharwell

StyleCop.Analyzers/StyleCop.Analyzers.Test.CSharp9/OrderingRules/SA1205CSharp9UnitTests.cs

@@ -3,9 +3,80 @@
 
 namespace StyleCop.Analyzers.Test.CSharp9.OrderingRules
 {
+    using System.Threading;
+    using System.Threading.Tasks;
+    using Microsoft.CodeAnalysis.Testing;
     using StyleCop.Analyzers.Test.CSharp8.OrderingRules;
+    using Xunit;
+    using static StyleCop.Analyzers.Test.Verifiers.StyleCopCodeFixVerifier<
+        StyleCop.Analyzers.OrderingRules.SA1205PartialElementsMustDeclareAccess,
+        StyleCop.Analyzers.OrderingRules.SA1205CodeFixProvider>;
 
     public partial class SA1205CSharp9UnitTests : SA1205CSharp8UnitTests
     {
+        [Fact]
+        [WorkItem(3971, "https://github.com/DotNetAnalyzers/StyleCopAnalyzers/issues/3971")]
+        public async Task TestPartialMethodWithPublicAccessModifierAsync()
+        {
+            var testCode = @"
+public partial class TestClass
+{
+    public partial int TestMethod();
+}
+
+public partial class TestClass
+{
+    public partial int TestMethod()
+    {
+        return 0;
+    }
+}
+";
+
+            await VerifyCSharpDiagnosticAsync(testCode, DiagnosticResult.EmptyDiagnosticResults, CancellationToken.None).ConfigureAwait(false);
+        }
+
+        [Fact]
+        [WorkItem(3971, "https://github.com/DotNetAnalyzers/StyleCopAnalyzers/issues/3971")]
+        public async Task TestPartialMethodWithoutAccessModifierAsync()
+        {
+            var testCode = @"
+public partial class TestClass
+{
+    partial void {|#0:TestMethod|}();
+}
+
+public partial class TestClass
+{
+    public partial void {|CS8799:TestMethod|}()
+    {
+    }
+}
+";
+
+            await VerifyCSharpDiagnosticAsync(testCode, DiagnosticResult.EmptyDiagnosticResults, CancellationToken.None).ConfigureAwait(false);
+        }
+
+        [Fact]
+        [WorkItem(3971, "https://github.com/DotNetAnalyzers/StyleCopAnalyzers/issues/3971")]
+        public async Task TestPartialMethodWithoutAccessModifierNonVoidAsync()
+        {
+            var testCode = @"
+public partial class TestClass
+{
+    partial int {|CS8796:TestMethod|}();
+}
+
+public partial class TestClass
+{
+    public partial int {|CS8799:TestMethod|}()
+    {
+        return 0;
+    }
+}
+";
+
+            await VerifyCSharpDiagnosticAsync(testCode, DiagnosticResult.EmptyDiagnosticResults, CancellationToken.None).ConfigureAwait(false);
+        }
     }
 }

documentation/SA1205.md

@@ -23,6 +23,13 @@ The partial element does not have an access modifier defined.
 
 A violation of this rule occurs when the partial elements does not have an access modifier defined.
 
+### Partial methods in C# 9
+
+C# 9 allows partial methods to have access modifiers and non-`void` return types. For compatibility across versions,
+SA1205 does not require these access modifiers to be included. In addition, the compiler enforces the presence of access
+modifiers when using the new features, so SA1205 will not apply duplicate diagnostics in cases where required modifiers
+are omitted.
+
 ## How to fix violations
 
 To fix an instance of this violation, specify an access modifier for the partial element.