Improve code fix for CS0621

Author: josefpihrt

src/CodeFixes/CSharp/CodeFixes/ModifiersCodeFixProvider.cs

@@ -11,6 +11,7 @@
 using Microsoft.CodeAnalysis.CSharp;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
 using Roslynator.CodeFixes;
+using Roslynator.CSharp.Syntax;
 
 namespace Roslynator.CSharp.CodeFixes
 {
@@ -218,7 +219,12 @@ public override async Task RegisterCodeFixesAsync(CodeFixContext context)
                     case CompilerDiagnosticIdentifiers.VirtualOrAbstractMembersCannotBePrivate:
                         {
                             if (Settings.IsEnabled(diagnostic.Id, CodeFixIdentifiers.RemoveVirtualModifier))
-                                ModifiersCodeFixRegistrator.RemoveModifier(context, diagnostic, node, SyntaxKind.VirtualKeyword, additionalKey: nameof(SyntaxKind.VirtualKeyword));
+                            {
+                                ModifierListInfo modifierInfo = SyntaxInfo.ModifierListInfo(node);
+
+                                if (modifierInfo.IsVirtual)
+                                   ModifiersCodeFixRegistrator.RemoveModifier(context, diagnostic, node, SyntaxKind.VirtualKeyword, additionalKey: nameof(SyntaxKind.VirtualKeyword));
+                            }
 
                             if (Settings.IsEnabled(diagnostic.Id, CodeFixIdentifiers.ChangeAccessibility))
                                 ModifiersCodeFixRegistrator.ChangeAccessibility(context, diagnostic, node, _publicOrInternalOrProtected);

src/CodeFixes/CodeFixes.xml

@@ -571,6 +571,7 @@
   <CodeFix Id="RCF0101" Identifier="RemoveVirtualModifier" Title="Remove 'virtual' modifier">
     <FixableDiagnosticIds>
       <Id>CS0549</Id>
+      <Id>CS0621</Id>
     </FixableDiagnosticIds>
   </CodeFix>
   <CodeFix Id="RCF0102" Identifier="MakeContainingClassUnsealed" Title="Make containing class unsealed">

src/Tests/CodeFixes.Tests/CS0621VirtualOrAbstractMembersCannotBePrivateTests.cs

@@ -50,5 +50,17 @@ private void M()
 }
 ", equivalenceKey: EquivalenceKey.Create(DiagnosticId, nameof(SyntaxKind.VirtualKeyword)));
         }
+
+        [Fact, Trait(Traits.CodeFix, CompilerDiagnosticIdentifiers.VirtualOrAbstractMembersCannotBePrivate)]
+        public async Task Test_RemoveAbstractModifier()
+        {
+            await VerifyNoFixAsync(@"
+abstract class C
+{
+    abstract void M();
+}
+",
+equivalenceKey: EquivalenceKey.Create(DiagnosticId, nameof(SyntaxKind.VirtualKeyword)));
+        }
     }
 }