Update SA1618 for partial methods

Author: sharwell

StyleCop.Analyzers/StyleCop.Analyzers.Test.CSharp9/DocumentationRules/SA1618CSharp9UnitTests.cs

@@ -3,9 +3,35 @@
 
 namespace StyleCop.Analyzers.Test.CSharp9.DocumentationRules
 {
+    using System.Threading;
+    using System.Threading.Tasks;
     using StyleCop.Analyzers.Test.CSharp8.DocumentationRules;
+    using Xunit;
+    using static StyleCop.Analyzers.Test.Verifiers.CustomDiagnosticVerifier<StyleCop.Analyzers.DocumentationRules.SA1618GenericTypeParametersMustBeDocumented>;
 
     public partial class SA1618CSharp9UnitTests : SA1618CSharp8UnitTests
     {
+        [Fact]
+        [WorkItem(3971, "https://github.com/DotNetAnalyzers/StyleCopAnalyzers/issues/3971")]
+        public async Task TestPartialMethodDeclarationMissingTypeParameterDocumentationAsync()
+        {
+            var testCode = @"
+/// <summary>
+/// Tests a partial method.
+/// </summary>
+public partial class TestClass
+{
+    /// <summary>Declaration.</summary>
+    public partial void TestMethod<{|#0:T|}>();
+
+    public partial void TestMethod<T>()
+    {
+    }
+}";
+
+            var expected = Diagnostic().WithLocation(0).WithArguments("T");
+
+            await VerifyCSharpDiagnosticAsync(testCode, new[] { expected }, CancellationToken.None).ConfigureAwait(false);
+        }
     }
 }

StyleCop.Analyzers/StyleCop.Analyzers.Test/DocumentationRules/SA1618UnitTests.cs

@@ -427,10 +427,10 @@ private void Test2<T>(int arg) { }
             await VerifyCSharpDiagnosticAsync(testCode, DiagnosticResult.EmptyDiagnosticResults, CancellationToken.None).ConfigureAwait(false);
         }
 
-        private static Task VerifyCSharpDiagnosticAsync(string source, DiagnosticResult expected, CancellationToken cancellationToken)
+        protected static Task VerifyCSharpDiagnosticAsync(string source, DiagnosticResult expected, CancellationToken cancellationToken)
             => VerifyCSharpDiagnosticAsync(source, new[] { expected }, cancellationToken);
 
-        private static Task VerifyCSharpDiagnosticAsync(string source, DiagnosticResult[] expected, CancellationToken cancellationToken)
+        protected static Task VerifyCSharpDiagnosticAsync(string source, DiagnosticResult[] expected, CancellationToken cancellationToken)
         {
             string contentClassWithTypeparamDoc = @"<?xml version=""1.0"" encoding=""utf-8"" ?>
 <TestClass>