Add C# language version check for read-only auto-properties.

Fixes #4696.

src/Compilers/CSharp/Portable/CSharpResources.Designer.cs

@@ -9161,6 +9161,15 @@ internal class CSharpResources {
             }
         }
         
+        /// <summary>
+        ///   Looks up a localized string similar to readonly automatically implemented properties.
+        /// </summary>
+        internal static string IDS_FeatureReadonlyAutoImplementedProperties {
+            get {
+                return ResourceManager.GetString("IDS_FeatureReadonlyAutoImplementedProperties", resourceCulture);
+            }
+        }
+        
         /// <summary>
         ///   Looks up a localized string similar to static classes.
         /// </summary>

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -219,6 +219,9 @@
   <data name="IDS_FeatureAutoImplementedProperties" xml:space="preserve">
     <value>automatically implemented properties</value>
   </data>
+  <data name="IDS_FeatureReadonlyAutoImplementedProperties" xml:space="preserve">
+    <value>readonly automatically implemented properties</value>
+  </data>
   <data name="IDS_FeatureObjectInitializer" xml:space="preserve">
     <value>object initializer</value>
   </data>

src/Compilers/CSharp/Portable/Errors/MessageID.cs

@@ -111,6 +111,7 @@ internal enum MessageID
         IDS_FeatureInterpolatedStrings = MessageBase + 12702,
         IDS_OperationCausedStackOverflow = MessageBase + 12703,
         IDS_AwaitInCatchAndFinally = MessageBase + 12704,
+        IDS_FeatureReadonlyAutoImplementedProperties = MessageBase + 12705,
     }
 
     // Message IDs may refer to strings that need to be localized.
@@ -171,6 +172,7 @@ internal static LanguageVersion RequiredVersion(this MessageID feature)
                 case MessageID.IDS_FeatureUsingStatic:
                 case MessageID.IDS_FeatureInterpolatedStrings:
                 case MessageID.IDS_AwaitInCatchAndFinally:
+                case MessageID.IDS_FeatureReadonlyAutoImplementedProperties:
                     return LanguageVersion.CSharp6;
 
                 // C# 5 features.

src/Compilers/CSharp/Portable/Symbols/Source/SourcePropertySymbol.cs

@@ -154,6 +154,7 @@ internal sealed class SourcePropertySymbol : PropertySymbol, IAttributeTargetSym
             {
                 var hasGetSyntax = getSyntax != null;
                 _isAutoProperty = notRegularProperty && hasGetSyntax;
+                bool isReadOnly = hasGetSyntax && setSyntax == null;
 
                 if (_isAutoProperty || hasInitializer)
                 {
@@ -165,7 +166,6 @@ internal sealed class SourcePropertySymbol : PropertySymbol, IAttributeTargetSym
                     }
 
                     string fieldName = GeneratedNames.MakeBackingFieldName(_sourceName);
-                    bool isReadOnly = hasGetSyntax && setSyntax == null;
                     _backingField = new SynthesizedBackingFieldSymbol(this,
                                                                           fieldName,
                                                                           isReadOnly,
@@ -175,7 +175,10 @@ internal sealed class SourcePropertySymbol : PropertySymbol, IAttributeTargetSym
 
                 if (notRegularProperty)
                 {
-                    Binder.CheckFeatureAvailability(location, MessageID.IDS_FeatureAutoImplementedProperties, diagnostics);
+                    Binder.CheckFeatureAvailability(location, 
+                                                    isReadOnly ? MessageID.IDS_FeatureReadonlyAutoImplementedProperties : 
+                                                                 MessageID.IDS_FeatureAutoImplementedProperties, 
+                                                    diagnostics);
                 }
             }
 

src/Compilers/CSharp/Test/Symbol/Symbols/Source/PropertyTests.cs

@@ -2784,5 +2784,39 @@ static void Main(string[] args)
     Diagnostic(ErrorCode.ERR_IllegalUnsafe, "Test").WithLocation(2, 14)
                 );
         }
+
+
+        [Fact, WorkItem(4696, "https://github.com/dotnet/roslyn/issues/4696")]
+        public void LangVersioAndReadonlyAutoProperty()
+        {
+            var source = @"
+public class Class1
+{
+    public Class1()
+    {
+        Prop1 = ""Test"";
+    }
+
+    public string Prop1 { get; }
+}
+
+abstract class Class2
+{
+    public abstract string Prop2 { get; }
+}
+
+interface I1
+{
+    string Prop3 { get; }
+}
+";
+
+            var comp = CreateCompilationWithMscorlib(source, parseOptions: CSharpParseOptions.Default.WithLanguageVersion(LanguageVersion.CSharp5));
+            comp.GetDeclarationDiagnostics().Verify(
+    // (9,19): error CS8026: Feature 'readonly automatically implemented properties' is not available in C# 5.  Please use language version 6 or greater.
+    //     public string Prop1 { get; }
+    Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion5, "Prop1").WithArguments("readonly automatically implemented properties", "6").WithLocation(9, 19)
+                );
+        }
     }
 }

src/Compilers/CSharp/Test/Symbol/Symbols/SymbolErrorTests.cs

@@ -1838,6 +1838,9 @@ class B : A
     // (8,34): error CS0112: A static member 'B.Q' cannot be marked as override, virtual, or abstract
     //     public static virtual object Q { get; }
     Diagnostic(ErrorCode.ERR_StaticNotVirtual, "Q").WithArguments("B.Q").WithLocation(8, 34),
+    // (8,34): error CS8026: Feature 'readonly automatically implemented properties' is not available in C# 5.  Please use language version 6 or greater.
+    //     public static virtual object Q { get; }
+    Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion5, "Q").WithArguments("readonly automatically implemented properties", "6").WithLocation(8, 34),
     // (9,37): error CS0112: A static member 'B.R' cannot be marked as override, virtual, or abstract
     //     internal static abstract object R { get; set; }
     Diagnostic(ErrorCode.ERR_StaticNotVirtual, "R").WithArguments("B.R").WithLocation(9, 37),