Improve analyzer SimplifyCoalesceExpression (RCS1143)

src/Analyzers/CSharp/Analysis/SimplifyCoalesceExpressionAnalyzer.cs

@@ -94,7 +94,7 @@ private static BinaryExpressionPart GetRedundantPart(
                     return BinaryExpressionPart.Left;
                 case SyntaxKind.DefaultExpression:
                     {
-                        if (IsDefaultOfReferenceType((DefaultExpressionSyntax)left, semanticModel, cancellationToken))
+                        if (IsDefaultOfReferenceOrNullableType((DefaultExpressionSyntax)left, semanticModel, cancellationToken))
                             return BinaryExpressionPart.Left;
 
                         break;
@@ -129,10 +129,12 @@ private static BinaryExpressionPart GetRedundantPart(
             switch (rightKind)
             {
                 case SyntaxKind.NullLiteralExpression:
-                    return BinaryExpressionPart.Right;
+                    {
+                        return BinaryExpressionPart.Right;
+                    }
                 case SyntaxKind.DefaultExpression:
                     {
-                        if (IsDefaultOfReferenceType((DefaultExpressionSyntax)right, semanticModel, cancellationToken))
+                        if (IsDefaultOfReferenceOrNullableType((DefaultExpressionSyntax)right, semanticModel, cancellationToken))
                             return BinaryExpressionPart.Right;
 
                         break;
@@ -148,12 +150,15 @@ private static BinaryExpressionPart GetRedundantPart(
             return BinaryExpressionPart.None;
         }
 
-        private static bool IsDefaultOfReferenceType(DefaultExpressionSyntax defaultExpression, SemanticModel semanticModel, CancellationToken cancellationToken)
+        private static bool IsDefaultOfReferenceOrNullableType(DefaultExpressionSyntax defaultExpression, SemanticModel semanticModel, CancellationToken cancellationToken)
         {
             TypeSyntax type = defaultExpression.Type;
 
             if (type != null)
             {
+                if (type.IsKind(SyntaxKind.NullableType))
+                    return true;
+
                 ITypeSymbol typeSymbol = semanticModel.GetTypeSymbol(type, cancellationToken);
 
                 if (typeSymbol?.IsErrorType() == false

src/Tests/Analyzers.Tests/RCS1143SimplifyCoalesceExpressionTests.cs

@@ -0,0 +1,50 @@
+// Copyright (c) Josef Pihrt. All rights reserved. Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System.Threading.Tasks;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CodeFixes;
+using Microsoft.CodeAnalysis.Diagnostics;
+using Roslynator.CSharp.CodeFixes;
+using Xunit;
+
+namespace Roslynator.CSharp.Analysis.Tests
+{
+    public class RCS1143SimplifyCoalesceExpressionTests : AbstractCSharpFixVerifier
+    {
+        public override DiagnosticDescriptor Descriptor { get; } = DiagnosticDescriptors.SimplifyCoalesceExpression;
+
+        public override DiagnosticAnalyzer Analyzer { get; } = new SimplifyCoalesceExpressionAnalyzer();
+
+        public override CodeFixProvider FixProvider { get; } = new BinaryExpressionCodeFixProvider();
+
+        [Fact, Trait(Traits.Analyzer, DiagnosticIdentifiers.SimplifyCoalesceExpression)]
+        public async Task Test_DefaultOfNullableType()
+        {
+            await VerifyDiagnosticAndFixAsync(@"
+class C
+{
+    void M()
+    {
+        C x = null;
+
+        int? y = x?.M2() [|?? default(int?)|];
+    }
+
+    int M2() => default;
+}
+", @"
+class C
+{
+    void M()
+    {
+        C x = null;
+
+        int? y = x?.M2();
+    }
+
+    int M2() => default;
+}
+");
+        }
+    }
+}