Merge pull request #17787 from OmarTawfik/error-on-extension-methods-in-legacy-code

Produce error on extension methods in language versions earlier than C# 3

src/Compilers/CSharp/Portable/Parser/LanguageParser.cs

@@ -4025,7 +4025,14 @@ private void ParseParameterModifiers(SyntaxListBuilder modifiers)
         {
             while (IsParameterModifier(this.CurrentToken.Kind))
             {
-                modifiers.Add(this.EatToken());
+                var modifier = this.EatToken();
+
+                if (modifier.Kind == SyntaxKind.ThisKeyword)
+                {
+                    modifier = CheckFeatureAvailability(modifier, MessageID.IDS_FeatureExtensionMethod);
+                }
+
+                modifiers.Add(modifier);
             }
         }
 

src/Compilers/CSharp/Test/Syntax/Parsing/ParserErrorMessageTests.cs

@@ -4736,6 +4736,23 @@ void Foo()
                 Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion2, "=>").WithArguments("lambda expression", "3"));
         }
 
+        [Fact]
+        public void ExtensionMethodsAreNotAvailableInEarlierCSharpVersions()
+        {
+            var code = @"
+ public static class Test
+ {
+     public static void DoSomething(this int x) { }
+ }";
+
+            ParseAndValidate(code, new CSharpParseOptions(LanguageVersion.CSharp2),
+                // (4,37): error CS8023: Feature 'extension method' is not available in C# 2.  Please use language version 3 or greater.
+                //      public static void DoSomething(this int x) { }
+                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion2, "this").WithArguments("extension method", "3").WithLocation(4, 37));
+
+            ParseAndValidate(code, new CSharpParseOptions(LanguageVersion.Latest));
+        }
+
         [Fact]
         public void ExceptionFilterBeforeVersionSix()
         {