Readonly members LangVersion checks (#34404)

* Add IDS_FeatureReadOnlyMembers

* Add CheckFeatureAvailability checks for readonly members

* Remove prototype comment

* Ensure LangVersion test is valid with default language version

* Fix DeclarationParsingTests

* Remove unnecessary comment

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

@@ -11482,6 +11482,15 @@ internal class CSharpResources {
             }
         }
         
+        /// <summary>
+        ///   Looks up a localized string similar to readonly members.
+        /// </summary>
+        internal static string IDS_FeatureReadOnlyMembers {
+            get {
+                return ResourceManager.GetString("IDS_FeatureReadOnlyMembers", resourceCulture);
+            }
+        }
+        
         /// <summary>
         ///   Looks up a localized string similar to readonly references.
         /// </summary>

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -219,6 +219,9 @@
   <data name="IDS_FeatureUnmanagedConstructedTypes" xml:space="preserve">
     <value>unmanaged constructed types</value>
   </data>
+  <data name="IDS_FeatureReadOnlyMembers" xml:space="preserve">
+    <value>readonly members</value>
+  </data>
   <data name="IDS_FeatureDefaultLiteral" xml:space="preserve">
     <value>default literal</value>
   </data>

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

@@ -175,6 +175,7 @@ internal enum MessageID
         IDS_FeatureStaticLocalFunctions = MessageBase + 12755,
         IDS_FeatureNameShadowingInNestedFunctions = MessageBase + 12756,
         IDS_FeatureUnmanagedConstructedTypes = MessageBase + 12757,
+        IDS_FeatureReadOnlyMembers = MessageBase + 12758,
     }
 
     // Message IDs may refer to strings that need to be localized.
@@ -279,6 +280,7 @@ internal static LanguageVersion RequiredVersion(this MessageID feature)
                 case MessageID.IDS_FeatureStaticLocalFunctions:
                 case MessageID.IDS_FeatureNameShadowingInNestedFunctions:
                 case MessageID.IDS_FeatureUnmanagedConstructedTypes: // semantic check
+                case MessageID.IDS_FeatureReadOnlyMembers:
                     return LanguageVersion.CSharp8;
 
                 // C# 7.3 features.

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

@@ -2388,6 +2388,9 @@ private MemberDeclarationSyntax ParseMemberDeclarationOrStatementCore(SyntaxKind
 
                     Debug.Assert(identifierOrThisOpt != null);
 
+                    // check availability of readonly members feature for indexers, properties and methods
+                    CheckForVersionSpecificModifiers(modifiers, SyntaxKind.ReadOnlyKeyword, MessageID.IDS_FeatureReadOnlyMembers);
+
                     if (identifierOrThisOpt.Kind == SyntaxKind.ThisKeyword)
                     {
                         return this.ParseIndexerDeclaration(attributes, modifiers, type, explicitInterfaceOpt, identifierOrThisOpt, typeParameterListOpt);
@@ -3401,6 +3404,9 @@ private AccessorDeclarationSyntax ParseAccessorDeclaration(bool isEvent)
                 this.ParseAttributeDeclarations(accAttrs);
                 this.ParseModifiers(accMods, forAccessors: true);
 
+                // check availability of readonly members feature for accessors
+                CheckForVersionSpecificModifiers(accMods, SyntaxKind.ReadOnlyKeyword, MessageID.IDS_FeatureReadOnlyMembers);
+
                 if (!isEvent)
                 {
                     if (accMods != null && accMods.Count > 0)
@@ -3965,6 +3971,9 @@ private void ParseParameterModifiers(SyntaxListBuilder modifiers)
 
             this.ParseMemberName(out explicitInterfaceOpt, out identifierOrThisOpt, out typeParameterList, isEvent: true);
 
+            // check availability of readonly members feature for custom events
+            CheckForVersionSpecificModifiers(modifiers, SyntaxKind.ReadOnlyKeyword, MessageID.IDS_FeatureReadOnlyMembers);
+
             // If we got an explicitInterfaceOpt but not an identifier, then we're in the special
             // case for ERR_ExplicitEventFieldImpl (see ParseMemberName for details).
             if (explicitInterfaceOpt != null && this.CurrentToken.Kind != SyntaxKind.OpenBraceToken)

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

@@ -752,6 +752,11 @@
         <target state="translated">operátor rozsahu</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureReadOnlyMembers">
+        <source>readonly members</source>
+        <target state="new">readonly members</target>
+        <note />
+      </trans-unit>
       <trans-unit id="IDS_FeatureRecursivePatterns">
         <source>recursive patterns</source>
         <target state="translated">rekurzivní vzory</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.de.xlf

@@ -752,6 +752,11 @@
         <target state="translated">Bereichsoperator</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureReadOnlyMembers">
+        <source>readonly members</source>
+        <target state="new">readonly members</target>
+        <note />
+      </trans-unit>
       <trans-unit id="IDS_FeatureRecursivePatterns">
         <source>recursive patterns</source>
         <target state="translated">Rekursive Muster</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.es.xlf

@@ -754,6 +754,11 @@
         <target state="translated">operador de intervalo</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureReadOnlyMembers">
+        <source>readonly members</source>
+        <target state="new">readonly members</target>
+        <note />
+      </trans-unit>
       <trans-unit id="IDS_FeatureRecursivePatterns">
         <source>recursive patterns</source>
         <target state="translated">patrones recursivos</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.fr.xlf

@@ -753,6 +753,11 @@
         <target state="translated">opérateur de plage</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureReadOnlyMembers">
+        <source>readonly members</source>
+        <target state="new">readonly members</target>
+        <note />
+      </trans-unit>
       <trans-unit id="IDS_FeatureRecursivePatterns">
         <source>recursive patterns</source>
         <target state="translated">modèles récursifs</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.it.xlf

@@ -752,6 +752,11 @@
         <target state="translated">operatore di intervallo</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureReadOnlyMembers">
+        <source>readonly members</source>
+        <target state="new">readonly members</target>
+        <note />
+      </trans-unit>
       <trans-unit id="IDS_FeatureRecursivePatterns">
         <source>recursive patterns</source>
         <target state="translated">criteri ricorsivi</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.ja.xlf

@@ -752,6 +752,11 @@
         <target state="translated">範囲演算子</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureReadOnlyMembers">
+        <source>readonly members</source>
+        <target state="new">readonly members</target>
+        <note />
+      </trans-unit>
       <trans-unit id="IDS_FeatureRecursivePatterns">
         <source>recursive patterns</source>
         <target state="translated">再帰的パターン</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.ko.xlf

@@ -752,6 +752,11 @@
         <target state="translated">범위 연산자</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureReadOnlyMembers">
+        <source>readonly members</source>
+        <target state="new">readonly members</target>
+        <note />
+      </trans-unit>
       <trans-unit id="IDS_FeatureRecursivePatterns">
         <source>recursive patterns</source>
         <target state="translated">재귀 패턴</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.pl.xlf

@@ -752,6 +752,11 @@
         <target state="translated">operator zakresu</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureReadOnlyMembers">
+        <source>readonly members</source>
+        <target state="new">readonly members</target>
+        <note />
+      </trans-unit>
       <trans-unit id="IDS_FeatureRecursivePatterns">
         <source>recursive patterns</source>
         <target state="translated">wzorce rekursywne</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.pt-BR.xlf

@@ -752,6 +752,11 @@
         <target state="translated">operador de intervalo</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureReadOnlyMembers">
+        <source>readonly members</source>
+        <target state="new">readonly members</target>
+        <note />
+      </trans-unit>
       <trans-unit id="IDS_FeatureRecursivePatterns">
         <source>recursive patterns</source>
         <target state="translated">padrões recursivos</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.ru.xlf

@@ -752,6 +752,11 @@
         <target state="translated">оператор range</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureReadOnlyMembers">
+        <source>readonly members</source>
+        <target state="new">readonly members</target>
+        <note />
+      </trans-unit>
       <trans-unit id="IDS_FeatureRecursivePatterns">
         <source>recursive patterns</source>
         <target state="translated">рекурсивные шаблоны</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.tr.xlf

@@ -753,6 +753,11 @@
         <target state="translated">aralık işleci</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureReadOnlyMembers">
+        <source>readonly members</source>
+        <target state="new">readonly members</target>
+        <note />
+      </trans-unit>
       <trans-unit id="IDS_FeatureRecursivePatterns">
         <source>recursive patterns</source>
         <target state="translated">özyinelemeli desenler</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.zh-Hans.xlf

@@ -753,6 +753,11 @@
         <target state="translated">范围运算符</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureReadOnlyMembers">
+        <source>readonly members</source>
+        <target state="new">readonly members</target>
+        <note />
+      </trans-unit>
       <trans-unit id="IDS_FeatureRecursivePatterns">
         <source>recursive patterns</source>
         <target state="translated">递归模式</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.zh-Hant.xlf

@@ -752,6 +752,11 @@
         <target state="translated">範圍運算子</target>
         <note />
       </trans-unit>
+      <trans-unit id="IDS_FeatureReadOnlyMembers">
+        <source>readonly members</source>
+        <target state="new">readonly members</target>
+        <note />
+      </trans-unit>
       <trans-unit id="IDS_FeatureRecursivePatterns">
         <source>recursive patterns</source>
         <target state="translated">遞迴模式</target>

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

@@ -940,8 +940,6 @@ public static void Main()
                 );
         }
 
-        // PROTOTYPE: readonly members features should require C# 8.0 or greater
-
         [Fact]
         public void ReadOnlyStructMethod()
         {
@@ -1780,5 +1778,53 @@ public struct S1
                 //         readonly remove {}
                 Diagnostic(ErrorCode.ERR_NoModifiersOnAccessor, "readonly").WithLocation(9, 9));
         }
+
+        [Fact]
+        public void ReadOnlyMembers_LangVersion()
+        {
+            var csharp = @"
+using System;
+
+public struct S
+{
+    public readonly void M() {}
+
+    public readonly int P1 => 42;
+    public int P2 { readonly get => 123; }
+    public int P3 { readonly set {} }
+
+    public readonly int this[int i] => i;
+    public int this[int i, int j] { readonly get => i + j; }
+
+    public readonly event Action<EventArgs> E { add {} remove {} }
+}
+";
+            var comp = CreateCompilation(csharp, parseOptions: TestOptions.Regular7_3);
+            comp.VerifyDiagnostics(
+                // (6,12): error CS8652: The feature 'readonly members' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //     public readonly void M1() {}
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "readonly").WithArguments("readonly members").WithLocation(6, 12),
+                // (8,12): error CS8652: The feature 'readonly members' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //     public readonly int P1 => 42;
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "readonly").WithArguments("readonly members").WithLocation(8, 12),
+                // (9,21): error CS8652: The feature 'readonly members' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //     public int P2 { readonly get => 123; }
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "readonly").WithArguments("readonly members").WithLocation(9, 21),
+                // (10,21): error CS8652: The feature 'readonly members' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //     public int P3 { readonly set {} }
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "readonly").WithArguments("readonly members").WithLocation(10, 21),
+                // (12,12): error CS8652: The feature 'readonly members' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //     public readonly int this[int i] => i;
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "readonly").WithArguments("readonly members").WithLocation(12, 12),
+                // (13,37): error CS8652: The feature 'readonly members' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //     public int this[int i, int j] { readonly get => i + j; }
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "readonly").WithArguments("readonly members").WithLocation(13, 37),
+                // (15,12): error CS8652: The feature 'readonly members' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //     public readonly event Action<EventArgs> E { add {} remove {} }
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "readonly").WithArguments("readonly members").WithLocation(15, 12));
+
+            comp = CreateCompilation(csharp);
+            comp.VerifyDiagnostics();
+        }
     }
 }

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

@@ -2733,7 +2733,7 @@ public void TestClassMethodWithPartial()
         public void TestStructMethodWithReadonly()
         {
             var text = "struct a { readonly void M() { } }";
-            var file = this.ParseFile(text);
+            var file = this.ParseFile(text, TestOptions.Regular);
 
             Assert.NotNull(file);
             Assert.Equal(1, file.Members.Count);
@@ -2780,7 +2780,7 @@ public void TestStructMethodWithReadonly()
         public void TestReadOnlyRefReturning()
         {
             var text = "struct a { readonly ref readonly int M() { } }";
-            var file = this.ParseFile(text);
+            var file = this.ParseFile(text, TestOptions.Regular);
 
             Assert.NotNull(file);
             Assert.Equal(1, file.Members.Count);
@@ -2830,7 +2830,7 @@ public void TestReadOnlyRefReturning()
         public void TestStructExpressionPropertyWithReadonly()
         {
             var text = "struct a { readonly int M => 42; }";
-            var file = this.ParseFile(text);
+            var file = this.ParseFile(text, TestOptions.Regular);
 
             Assert.NotNull(file);
             Assert.Equal(1, file.Members.Count);
@@ -2871,7 +2871,7 @@ public void TestStructExpressionPropertyWithReadonly()
         public void TestStructGetterPropertyWithReadonly()
         {
             var text = "struct a { int P { readonly get { return 42; } } }";
-            var file = this.ParseFile(text);
+            var file = this.ParseFile(text, TestOptions.Regular);
 
             Assert.NotNull(file);
             Assert.Equal(1, file.Members.Count);