Fix MA0050 code fixer producing an extension local function (CS1106)

src/Meziantou.Analyzer.CodeFixers/Rules/ValidateArgumentsCorrectlyFixer.cs

@@ -58,7 +58,7 @@ private static async Task<Document> Refactor(Document document, Diagnostic diagn
         // Create local function
         var returnTypeSyntax = generator.TypeExpression(symbol.ReturnType);
         var localFunctionSyntaxNode = LocalFunctionStatement((TypeSyntax)returnTypeSyntax, symbol.Name);
-        localFunctionSyntaxNode = localFunctionSyntaxNode.WithParameterList(CopyParametersWithoutDefaultValues(methodSyntaxNode.ParameterList));
+        localFunctionSyntaxNode = localFunctionSyntaxNode.WithParameterList(CreateLocalFunctionParameterList(methodSyntaxNode.ParameterList));
         if (methodSyntaxNode.Modifiers.Any(m => m.IsKind(SyntaxKind.AsyncKeyword)))
         {
             localFunctionSyntaxNode = localFunctionSyntaxNode.AddModifiers(Token(SyntaxKind.AsyncKeyword));
@@ -95,9 +95,13 @@ private static async Task<Document> Refactor(Document document, Diagnostic diagn
         return editor.GetChangedDocument();
     }
 
-    private static ParameterListSyntax CopyParametersWithoutDefaultValues(ParameterListSyntax parameterListSyntax)
+    private static ParameterListSyntax CreateLocalFunctionParameterList(ParameterListSyntax parameterListSyntax)
     {
-        return ParameterList(SeparatedList(parameterListSyntax.Parameters.Select(p => p.WithDefault(null))));
+        // Default values are useless as the local function is always called with all the arguments.
+        // The "this" modifier must be removed as a local function cannot be an extension method (CS1106).
+        return ParameterList(SeparatedList(parameterListSyntax.Parameters.Select(p => p
+            .WithDefault(null)
+            .WithModifiers(TokenList(p.Modifiers.Where(modifier => !modifier.IsKind(SyntaxKind.ThisKeyword)))))));
     }
 
     private static ParameterListSyntax RemoveEnumeratorCancellationAttribute(ParameterListSyntax parameterListSyntax, SemanticModel semanticModel, CancellationToken cancellationToken)

tests/Meziantou.Analyzer.Test/Rules/ValidateArgumentsCorrectlyAnalyzerTests.cs

@@ -465,4 +465,47 @@ await CreateProjectBuilder()
               .ShouldFixCodeWith(CodeFix)
               .ValidateAsync();
     }
+
+    [Fact]
+    public async Task ReportDiagnostic_ExtensionMethod()
+    {
+        const string SourceCode = """
+            using System;
+            using System.Collections.Generic;
+            static class TypeName
+            {
+                public static IEnumerable<int> [|A|](this string a)
+                {
+                    if (a is null)
+                        throw new ArgumentNullException(nameof(a));
+
+                    yield return 1;
+                }
+            }
+            """;
+
+        const string CodeFix = """
+            using System;
+            using System.Collections.Generic;
+            static class TypeName
+            {
+                public static IEnumerable<int> A(this string a)
+                {
+                    if (a is null)
+                        throw new ArgumentNullException(nameof(a));
+
+                    return A(a);
+                    IEnumerable<int> A(string a)
+                    {
+                        yield return 1;
+                    }
+                }
+            }
+            """;
+
+        await CreateProjectBuilder()
+              .WithSourceCode(SourceCode)
+              .ShouldFixCodeWith(CodeFix)
+              .ValidateAsync();
+    }
 }