Don't add accessibility modifiers to partial classes in new documents

src/Analyzers/CSharp/Tests/AddAccessibilityModifiers/AddAccessibilityModifiersTests.cs

@@ -548,5 +548,38 @@ internal struct S1 { }
                 LanguageVersion = LanguageVersion.CSharp10,
             }.RunAsync();
         }
+
+        [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsAddAccessibilityModifiers)]
+        [WorkItem(55703, "https://github.com/dotnet/roslyn/issues/55703")]
+        public async Task TestPartial_WithExistingModifier()
+        {
+            var source = @"
+partial class [|C|]
+{
+}
+
+public partial class C
+{
+}
+";
+            var fixedSource = @"
+public partial class C
+{
+}
+
+public partial class C
+{
+}
+";
+
+            var test = new VerifyCS.Test
+            {
+                TestCode = source,
+                FixedCode = fixedSource,
+                LanguageVersion = CodeAnalysis.CSharp.LanguageVersion.Preview,
+            };
+
+            await test.RunAsync();
+        }
     }
 }

src/EditorFeatures/CSharpTest/Formatting/CSharpNewDocumentFormattingServiceTests.cs

@@ -8,8 +8,8 @@
 using Microsoft.CodeAnalysis.CSharp;
 using Microsoft.CodeAnalysis.CSharp.CodeStyle;
 using Microsoft.CodeAnalysis.Editor.UnitTests.Workspaces;
-using Microsoft.CodeAnalysis.Notification;
 using Microsoft.CodeAnalysis.Test.Utilities.Formatting;
+using Roslyn.Test.Utilities;
 using Xunit;
 
 namespace Microsoft.CodeAnalysis.Editor.CSharp.UnitTests.Formatting
@@ -171,6 +171,49 @@ internal class C
             });
         }
 
+        [Fact]
+        [WorkItem(55703, "https://github.com/dotnet/roslyn/issues/55703")]
+        public async Task TestAccessibilityModifiers_IgnoresPartial()
+        {
+            await TestAsync(
+                testCode: @"using System;
+
+namespace Goo
+{
+    class E
+    {
+    }
+
+    partial class C
+    {
+    }
+
+    class D
+    {
+    }
+}",
+                expected: @"using System;
+
+namespace Goo
+{
+    internal class E
+    {
+    }
+
+    partial class C
+    {
+    }
+
+    internal class D
+    {
+    }
+}",
+                options: new[]
+                {
+                    (CodeStyleOptions2.RequireAccessibilityModifiers, new CodeStyleOption2<AccessibilityModifiersRequired>(AccessibilityModifiersRequired.Always, NotificationOption2.Error))
+                });
+        }
+
         [Fact]
         public async Task TestUsingDirectivePlacement()
         {

src/Features/CSharp/Portable/Formatting/CSharpAccessibilityModifiersNewDocumentFormattingProvider.cs

@@ -47,6 +47,23 @@ public async Task<Document> FormatNewDocumentAsync(Document document, Document?
                 if (!service.ShouldUpdateAccessibilityModifier(syntaxFacts, declaration, accessibilityPreferences.Value, out _))
                     continue;
 
+                // Since we format each document as they are added to a project we can't assume we know about all
+                // of the files that are coming, so we have to opt out of changing partial classes. This especially
+                // manifests when creating new projects as we format before we have a project at all, so we could get a
+                // situation like this:
+                //
+                // File1.cs:
+                //    partial class C { }
+                // File2.cs:
+                //    public partial class C { }
+                //
+                // When we see File1, we don't know about File2, so would add an internal modifier, which would result in a compile
+                // error.
+                var modifiers = syntaxFacts.GetModifiers(declaration);
+                syntaxFacts.GetAccessibilityAndModifiers(modifiers, out _, out var declarationModifiers, out _);
+                if (declarationModifiers.IsPartial)
+                    continue;
+
                 var type = semanticModel.GetDeclaredSymbol(declaration, cancellationToken);
                 if (type == null)
                     continue;

src/VisualStudio/IntegrationTest/IntegrationTests/CSharp/CSharpWinForms.cs

@@ -105,7 +105,7 @@ private void ExecuteWhenButtonClicked(object sender, EventArgs e)
     }", codeFileActualText);
         }
 
-        [WpfFact(Skip = "https://github.com/dotnet/roslyn/issues/55703"), Trait(Traits.Feature, Traits.Features.WinForms)]
+        [WpfFact, Trait(Traits.Feature, Traits.Features.WinForms)]
         public void RenameControl()
         {
             var project = new ProjectUtils.Project(ProjectName);