Check language version on relaxed ordering of parameter modifiers

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

@@ -3670,6 +3670,11 @@ private void ParseParameterModifiers(SyntaxListBuilder modifiers)
                 {
                     case SyntaxKind.ThisKeyword:
                         modifier = CheckFeatureAvailability(modifier, MessageID.IDS_FeatureExtensionMethod);
+                        if (this.CurrentToken.Kind == SyntaxKind.RefKeyword ||
+                            this.CurrentToken.Kind == SyntaxKind.InKeyword)
+                        {
+                            modifier = CheckFeatureAvailability(modifier, MessageID.IDS_FeatureRefExtensionMethods);
+                        }
                         break;
 
                     case SyntaxKind.RefKeyword:

src/Compilers/CSharp/Test/Semantic/Semantics/RefExtensionMethodsTests.cs

@@ -2346,33 +2346,61 @@ public void UsingRefExtensionMethodsBeforeVersion7_2ProducesDiagnostics_InSyntax
             var code = @"
 public static class Ext
 {
-    public static void Print(in this int p)
-    {
-        System.Console.WriteLine(p);
-    }
+    public static void Print0(in this int p)
+        => System.Console.Write(p);
+    public static void Print1(this in int p)
+        => System.Console.Write(p);
+    public static void Print2(ref this int p)
+        => System.Console.Write(p);
+    public static void Print3(this ref int p)
+        => System.Console.Write(p);
 }
 public static class Program
 {
     public static void Main()
     {
         int p = 5;
-        p.Print();
+        p.Print0();
+        p.Print1();
+        p.Print2();
+        p.Print3();
     }
 }";
 
             CreateCompilationWithMscorlib40AndSystemCore(code, parseOptions: CSharpParseOptions.Default.WithLanguageVersion(LanguageVersion.CSharp7_1)).VerifyDiagnostics(
-                // (4,30): error CS8302: Feature 'readonly references' is not available in C# 7.1. Please use language version 7.2 or greater.
-                //     public static void Print(in this int p)
-                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_1, "in").WithArguments("readonly references", "7.2").WithLocation(4, 30),
-                // (4,30): error CS8302: Feature 'ref extension methods' is not available in C# 7.1. Please use language version 7.2 or greater.
-                //     public static void Print(in this int p)
-                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_1, "in").WithArguments("ref extension methods", "7.2").WithLocation(4, 30),
-                // (14,9): error CS8302: Feature 'ref extension methods' is not available in C# 7.1. Please use language version 7.2 or greater.
-                //         p.Print();
-                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_1, "p").WithArguments("ref extension methods", "7.2").WithLocation(14, 9)
-            );
-
-            CompileAndVerify(code, expectedOutput: "5");
+                // (4,31): error CS8302: Feature 'readonly references' is not available in C# 7.1. Please use language version 7.2 or greater.
+                //     public static void Print0(in this int p)
+                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_1, "in").WithArguments("readonly references", "7.2").WithLocation(4, 31),
+                // (4,31): error CS8302: Feature 'ref extension methods' is not available in C# 7.1. Please use language version 7.2 or greater.
+                //     public static void Print0(in this int p)
+                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_1, "in").WithArguments("ref extension methods", "7.2").WithLocation(4, 31),
+                // (6,31): error CS8302: Feature 'ref extension methods' is not available in C# 7.1. Please use language version 7.2 or greater.
+                //     public static void Print1(this in int p)
+                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_1, "this").WithArguments("ref extension methods", "7.2").WithLocation(6, 31),
+                // (6,36): error CS8302: Feature 'readonly references' is not available in C# 7.1. Please use language version 7.2 or greater.
+                //     public static void Print1(this in int p)
+                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_1, "in").WithArguments("readonly references", "7.2").WithLocation(6, 36),
+                // (8,31): error CS8302: Feature 'ref extension methods' is not available in C# 7.1. Please use language version 7.2 or greater.
+                //     public static void Print2(ref this int p)
+                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_1, "ref").WithArguments("ref extension methods", "7.2").WithLocation(8, 31),
+                // (10,31): error CS8302: Feature 'ref extension methods' is not available in C# 7.1. Please use language version 7.2 or greater.
+                //     public static void Print3(this ref int p)
+                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_1, "this").WithArguments("ref extension methods", "7.2").WithLocation(10, 31),
+                // (18,9): error CS8302: Feature 'ref extension methods' is not available in C# 7.1. Please use language version 7.2 or greater.
+                //         p.Print0();
+                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_1, "p").WithArguments("ref extension methods", "7.2").WithLocation(18, 9),
+                // (19,9): error CS8302: Feature 'ref extension methods' is not available in C# 7.1. Please use language version 7.2 or greater.
+                //         p.Print1();
+                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_1, "p").WithArguments("ref extension methods", "7.2").WithLocation(19, 9),
+                // (20,9): error CS8302: Feature 'ref extension methods' is not available in C# 7.1. Please use language version 7.2 or greater.
+                //         p.Print2();
+                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_1, "p").WithArguments("ref extension methods", "7.2").WithLocation(20, 9),
+                // (21,9): error CS8302: Feature 'ref extension methods' is not available in C# 7.1. Please use language version 7.2 or greater.
+                //         p.Print3();
+                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_1, "p").WithArguments("ref extension methods", "7.2").WithLocation(21, 9)
+                );
+
+            CompileAndVerify(code, expectedOutput: "5555");
         }
 
         [Fact]