Update SA1131 to support default literal expressions

Fixes #2675

Author: sharwell

StyleCop.Analyzers/StyleCop.Analyzers.Test.CSharp7/ReadabilityRules/SA1131CSharp7UnitTests.cs

@@ -3,9 +3,72 @@
 
 namespace StyleCop.Analyzers.Test.CSharp7.ReadabilityRules
 {
+    using System.Threading;
+    using System.Threading.Tasks;
+    using Microsoft.CodeAnalysis;
+    using Microsoft.CodeAnalysis.CSharp;
     using StyleCop.Analyzers.Test.ReadabilityRules;
+    using TestHelper;
+    using Xunit;
 
     public class SA1131CSharp7UnitTests : SA1131UnitTests
     {
+        [Theory]
+        [InlineData("==", "==")]
+        [InlineData("!=", "!=")]
+        [WorkItem(2675, "https://github.com/DotNetAnalyzers/StyleCopAnalyzers/issues/2675")]
+        public async Task TestDefaultLiteralStructComparismOutsideIfAsync(string oldOperator, string newOperator)
+        {
+            var testCode = $@"
+using System;
+public class TypeName
+{{
+    public void Test()
+    {{
+        TestStruct i = default;
+        bool b = default {oldOperator} i;
+    }}
+}}
+
+struct TestStruct 
+{{
+    public static bool operator == (TestStruct a, TestStruct b) {{ return true; }}
+    public static bool operator != (TestStruct a, TestStruct b) {{ return false; }}
+}}
+";
+            var fixedCode = $@"
+using System;
+public class TypeName
+{{
+    public void Test()
+    {{
+        TestStruct i = default;
+        bool b = i {newOperator} default;
+    }}
+}}
+
+struct TestStruct 
+{{
+    public static bool operator == (TestStruct a, TestStruct b) {{ return true; }}
+    public static bool operator != (TestStruct a, TestStruct b) {{ return false; }}
+}}
+";
+            DiagnosticResult[] expected =
+            {
+                this.CSharpDiagnostic().WithLocation(8, 18),
+            };
+            await this.VerifyCSharpDiagnosticAsync(testCode, expected, CancellationToken.None).ConfigureAwait(false);
+            await this.VerifyCSharpDiagnosticAsync(fixedCode, EmptyDiagnosticResults, CancellationToken.None).ConfigureAwait(false);
+            await this.VerifyCSharpFixAsync(testCode, fixedCode, cancellationToken: CancellationToken.None).ConfigureAwait(false);
+        }
+
+        protected override Project ApplyCompilationOptions(Project project)
+        {
+            var newProject = base.ApplyCompilationOptions(project);
+
+            var parseOptions = (CSharpParseOptions)newProject.ParseOptions;
+
+            return newProject.WithParseOptions(parseOptions.WithLanguageVersion(LanguageVersion.Latest));
+        }
     }
 }

StyleCop.Analyzers/StyleCop.Analyzers/ReadabilityRules/SA1131UseReadableConditions.cs

@@ -9,6 +9,7 @@ namespace StyleCop.Analyzers.ReadabilityRules
     using Microsoft.CodeAnalysis.CSharp;
     using Microsoft.CodeAnalysis.CSharp.Syntax;
     using Microsoft.CodeAnalysis.Diagnostics;
+    using StyleCop.Analyzers.Lightup;
 
     /// <summary>
     /// A comparison was made between a variable and a literal or constant value, and the variable appeared on the
@@ -68,7 +69,7 @@ private static void HandleBinaryExpression(SyntaxNodeAnalysisContext context)
         private static bool IsLiteral(ExpressionSyntax expression, SemanticModel semanticModel)
         {
             // Default expressions are most of the time constants, but not for default(MyStruct).
-            if (expression.IsKind(SyntaxKind.DefaultExpression))
+            if (expression.IsKind(SyntaxKind.DefaultExpression) || expression.IsKind(SyntaxKindEx.DefaultLiteralExpression))
             {
                 return true;
             }