add test demonstrating legal and illegal combinations of ref/out and …

…properties in which the new message still makes sense

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

@@ -8832,31 +8832,44 @@ static void Main()
         /// Ref auto-properties are not supported.
         /// </summary>
         [Fact]
-        public void RefAutoProperty_03()
+        public void RefAndOutProperties()
         {
             var source =
-@"using System;
-class C
-{
-    public static string P0 { get; }
-    public static string P1 { get; set; }
-}
-class Program
+@"
+ref string S = ref C.S;    //CS0206
+var c = new C();
+var str = C.M(ref c.N);    //ref prop
+var str2 = C.M(ref c.N2);  //CS0206
+var str3 = C.M2(out c.N);  //ref prop
+var str4 = C.M2(out c.N2); //CS0206
+ref struct C
 {
-    static void Main()
+    public static string S { get; set; }
+    private ref int n;
+    public ref int N => ref n;
+    public int N2 { get; set; }
+    public static string M(ref int number)
     {
-        ref var p0 = ref C.P0;
-        ref var p1 = ref C.P1;
+        return number.ToString();
     }
-}";
-            var comp = CreateCompilation(source);
+    public static string M2(out int number)
+    {
+        number = 42;
+        return number.ToString();
+    }
+}
+";
+            var comp = CreateCompilation(source, targetFramework: TargetFramework.Net70);
             comp.VerifyEmitDiagnostics(
-                // (11,26): error CS8145: Auto-implemented properties cannot return by reference
-                //         ref var p0 = ref C.P0;
-                Diagnostic(ErrorCode.ERR_AutoPropertyCannotBeRefReturning, "C.P0").WithLocation(11, 26),
-                // (12,26): error CS8145: Auto-implemented properties cannot return by reference
-                //         ref var p1 = ref C.P1;
-                Diagnostic(ErrorCode.ERR_AutoPropertyCannotBeRefReturning, "C.P1").WithLocation(12, 26)
+                // (2,20): error CS0206: An indexer or auto-implemented property may not be used as an out or ref value
+                // ref string S = ref C.S; //CS0206
+                Diagnostic(ErrorCode.ERR_RefProperty, "C.S").WithLocation(2, 20),
+                // (5,20): error CS0206: An indexer or auto-implemented property may not be used as an out or ref value
+                // var str2 = C.M(ref c.N2);
+                Diagnostic(ErrorCode.ERR_RefProperty, "c.N2").WithLocation(5, 20),
+                // (7,21): error CS0206: An indexer or auto-implemented property may not be used as an out or ref value
+                // var str4 = C.M2(out c.N2);
+                Diagnostic(ErrorCode.ERR_RefProperty, "c.N2").WithLocation(7, 21)
             );
         }