Update SA1131 for nullable reference types

Author: sharwell

StyleCop.Analyzers/StyleCop.Analyzers.Test.CSharp8/ReadabilityRules/SA1131CSharp8UnitTests.cs

@@ -3,9 +3,37 @@
 
 namespace StyleCop.Analyzers.Test.CSharp8.ReadabilityRules
 {
+    using System.Threading;
+    using System.Threading.Tasks;
+    using Microsoft.CodeAnalysis.Testing;
     using StyleCop.Analyzers.Test.CSharp7.ReadabilityRules;
+    using Xunit;
+    using static StyleCop.Analyzers.Test.Verifiers.StyleCopCodeFixVerifier<
+        StyleCop.Analyzers.ReadabilityRules.SA1131UseReadableConditions,
+        StyleCop.Analyzers.ReadabilityRules.SA1131CodeFixProvider>;
 
     public partial class SA1131CSharp8UnitTests : SA1131CSharp7UnitTests
     {
+        [Fact]
+        [WorkItem(3006, "https://github.com/DotNetAnalyzers/StyleCopAnalyzers/issues/3006")]
+        public async Task TestPatternMatchingNullChecksAsync()
+        {
+            var testCode = @"#nullable enable
+public class TestClass
+{
+    public bool IsNull(string? value)
+    {
+        return value is null;
+    }
+
+    public bool IsNotNull(string? value)
+    {
+        return value is { };
+    }
+}
+";
+
+            await VerifyCSharpDiagnosticAsync(testCode, DiagnosticResult.EmptyDiagnosticResults, CancellationToken.None).ConfigureAwait(false);
+        }
     }
 }

documentation/SA1131.md

@@ -47,6 +47,7 @@ For the purposes of this rule, a *literal or constant value* is any of the follo
 * `default(T)` (for any type `T`)
 * Any expression which evaluates to a constant value at compile time
 * A reference to a `static readonly` field, such as `IntPtr.Zero`
+* A pattern
 
 A *variable value* is any expression which is not considered a literal or constant value.