Readonly members warnings and semantic errors (#34022)

* readonly members warnings WIP

* Remove dead code

* Readonly members semantic checks. Warn on implicit copy of 'this'.

* Add to readonly ref readonly test

* Warn on implicit copies for get property access

* Don't warn on calls to base methods

* Add test for setter accesses in a readonly member

* Allow readonly setter accesses in readonly methods

* Test increment, compound assignment, and field-like events

* Fixes from feedback

* Add mutating readonly event accessors test

* Expand on readonly base members test

* Check for implicit copies in foreach/using/deconstruction scenarios

* Split base method tests into multiple tests for clarity

* Test remaining cases

* Fix whitespace in test

* Add ReadOnlyMethod_RefLocal

* Fixes from Fred's feedback

src/Compilers/CSharp/Portable/Binder/Binder.ValueChecks.cs

@@ -438,8 +438,6 @@ internal bool CheckValueKind(SyntaxNode node, BoundExpression expr, BindValueKin
                     return CheckLocalValueKind(node, local, valueKind, checkingReceiver, diagnostics);
 
                 case BoundKind.ThisReference:
-                    var thisref = (BoundThisReference)expr;
-
                     // `this` is never ref assignable
                     if (RequiresRefAssignableVariable(valueKind))
                     {
@@ -457,14 +455,12 @@ internal bool CheckValueKind(SyntaxNode node, BoundExpression expr, BindValueKin
                     // SPEC: of a struct, it is classified as a variable. 
 
                     // Note: RValueOnly is checked at the beginning of this method. Since we are here we need more than readable.
-                    //"this" is readonly in members of readonly structs, unless we are in a constructor.
-                    if (!thisref.Type.IsValueType ||
-                            (RequiresAssignableVariable(valueKind) &&
-                             thisref.Type.IsReadOnly &&
-                             (this.ContainingMemberOrLambda as MethodSymbol)?.MethodKind != MethodKind.Constructor))
+                    // "this" is readonly in members marked "readonly" and in members of readonly structs, unless we are in a constructor.
+                    var isValueType = ((BoundThisReference)expr).Type.IsValueType;
+                    if (!isValueType || (RequiresAssignableVariable(valueKind) && (this.ContainingMemberOrLambda as MethodSymbol)?.IsEffectivelyReadOnly == true))
                     {
                         // CONSIDER: the Dev10 name has angle brackets (i.e. "<this>")
-                        Error(diagnostics, GetThisLvalueError(valueKind), node, ThisParameterSymbol.SymbolName);
+                        Error(diagnostics, GetThisLvalueError(valueKind, isValueType), node, ThisParameterSymbol.SymbolName);
                         return false;
                     }
 
@@ -994,7 +990,8 @@ private bool CheckPropertyValueKind(SyntaxNode node, BoundExpression expr, BindV
 
                     ReportDiagnosticsIfObsolete(diagnostics, setMethod, node, receiver?.Kind == BoundKind.BaseReference);
 
-                    if (RequiresVariableReceiver(receiver, setMethod) && !CheckIsValidReceiverForVariable(node, receiver, BindValueKind.Assignable, diagnostics))
+                    var setValueKind = setMethod.IsEffectivelyReadOnly ? BindValueKind.RValue : BindValueKind.Assignable;
+                    if (RequiresVariableReceiver(receiver, setMethod) && !CheckIsValidReceiverForVariable(node, receiver, setValueKind, diagnostics))
                     {
                         return false;
                     }
@@ -1041,6 +1038,7 @@ private bool CheckPropertyValueKind(SyntaxNode node, BoundExpression expr, BindV
                         return false;
                     }
 
+                    CheckImplicitThisCopyInReadOnlyMember(receiver, getMethod, diagnostics);
                     ReportDiagnosticsIfObsolete(diagnostics, getMethod, node, receiver?.Kind == BoundKind.BaseReference);
                 }
             }
@@ -1518,7 +1516,7 @@ private static void ReportReadonlyLocalError(SyntaxNode node, LocalSymbol local,
             Error(diagnostics, ReadOnlyLocalErrors[index], node, local, cause.Localize());
         }
 
-        static private ErrorCode GetThisLvalueError(BindValueKind kind)
+        static private ErrorCode GetThisLvalueError(BindValueKind kind, bool isValueType)
         {
             switch (kind)
             {
@@ -1533,7 +1531,7 @@ static private ErrorCode GetThisLvalueError(BindValueKind kind)
                     return ErrorCode.ERR_InvalidAddrOp;
 
                 case BindValueKind.IncrementDecrement:
-                    return ErrorCode.ERR_IncrementLvalueExpected;
+                    return isValueType ? ErrorCode.ERR_AssgReadonlyLocal : ErrorCode.ERR_IncrementLvalueExpected;
 
                 case BindValueKind.RefReturn:
                 case BindValueKind.ReadonlyRef:
@@ -2969,7 +2967,7 @@ internal enum AddressKind
 
                     if (!IsAnyReadOnly(addressKind) && method.IsEffectivelyReadOnly)
                     {
-                        return method.MethodKind == MethodKind.Constructor;
+                        return false;
                     }
 
                     return true;

src/Compilers/CSharp/Portable/Binder/Binder_Deconstruct.cs

@@ -142,6 +142,11 @@ internal BoundExpression BindDeconstruction(AssignmentExpressionSyntax node, Dia
                                     checkedVariables,
                                     out conversion);
 
+            if (conversion.Method != null)
+            {
+                CheckImplicitThisCopyInReadOnlyMember(boundRHS, conversion.Method, diagnostics);
+            }
+
             FailRemainingInferencesAndSetValEscape(checkedVariables, diagnostics, rightEscape);
 
             var lhsTuple = DeconstructionVariablesAsTuple(left, checkedVariables, diagnostics, ignoreDiagnosticsFromTuple: diagnostics.HasAnyErrors() || !resultIsUsed);

src/Compilers/CSharp/Portable/Binder/Binder_Expressions.cs

@@ -5775,6 +5775,9 @@ private bool IsUsingAliasInScope(string name)
                             }
                             else if ((object)leftType != null)
                             {
+                                // NB: We don't know if we really only need RValue access, or if we are actually
+                                // passing the receiver implicitly by ref (e.g. in a struct instance method invocation).
+                                // These checks occur later.
                                 boundLeft = CheckValue(boundLeft, BindValueKind.RValue, diagnostics);
                                 return BindInstanceMemberAccess(node, right, boundLeft, rightName, rightArity, typeArgumentsSyntax, typeArguments, invoked, indexed, diagnostics);
                             }

src/Compilers/CSharp/Portable/Binder/Binder_Invocation.cs

@@ -1010,6 +1010,8 @@ private void CheckRestrictedTypeReceiver(BoundExpression expression, Compilation
             // (i.e. the first argument, if invokedAsExtensionMethod).
             var gotError = MemberGroupFinalValidation(receiver, method, expression, diagnostics, invokedAsExtensionMethod);
 
+            CheckImplicitThisCopyInReadOnlyMember(receiver, method, diagnostics);
+
             if (invokedAsExtensionMethod)
             {
                 BoundExpression receiverArgument = analyzedArguments.Argument(0);
@@ -1135,30 +1137,27 @@ private void CheckRestrictedTypeReceiver(BoundExpression expression, Compilation
                         argsToParamsOpt: argsToParams, resultKind: LookupResultKind.Viable, binderOpt: this, type: returnType, hasErrors: gotError);
         }
 
-        private bool IsBindingModuleLevelAttribute()
+        /// <summary>
+        /// Returns false if an implicit 'this' copy will occur due to an instance member invocation in a readonly member.
+        /// </summary>
+        internal bool CheckImplicitThisCopyInReadOnlyMember(BoundExpression receiver, MethodSymbol method, DiagnosticBag diagnostics)
         {
-            if ((this.Flags & BinderFlags.InContextualAttributeBinder) == 0)
+            // For now we are warning only in implicit copy scenarios that are only possible with readonly members.
+            // Eventually we will warn on implicit value copies in more scenarios. See https://github.com/dotnet/roslyn/issues/33968.
+            if (receiver is BoundThisReference &&
+                receiver.Type.IsValueType &&
+                ContainingMemberOrLambda is MethodSymbol containingMethod &&
+                containingMethod.IsEffectivelyReadOnly &&
+                // Ignore calls to base members.
+                TypeSymbol.Equals(containingMethod.ContainingType, method.ContainingType, TypeCompareKind.ConsiderEverything) &&
+                !method.IsEffectivelyReadOnly &&
+                !method.IsStatic)
             {
+                Error(diagnostics, ErrorCode.WRN_ImplicitCopyInReadOnlyMember, receiver.Syntax, method.Name, ThisParameterSymbol.SymbolName);
                 return false;
             }
 
-            var current = this;
-
-            do
-            {
-                var contextualAttributeBinder = current as ContextualAttributeBinder;
-
-                if (contextualAttributeBinder != null)
-                {
-                    return (object)contextualAttributeBinder.AttributeTarget != null &&
-                           contextualAttributeBinder.AttributeTarget.Kind == SymbolKind.NetModule;
-                }
-
-                current = current.Next;
-            }
-            while (current != null);
-
-            throw ExceptionUtilities.Unreachable;
+            return true;
         }
 
         /// <param name="node">Invocation syntax node.</param>

src/Compilers/CSharp/Portable/Binder/ForEachLoopBinder.cs

@@ -207,6 +207,10 @@ private BoundForEachStatement BindForEachPartsWorker(DiagnosticBag diagnostics,
 
             AwaitableInfo awaitInfo = null;
             MethodSymbol getEnumeratorMethod = builder.GetEnumeratorMethod;
+            if (getEnumeratorMethod != null)
+            {
+                originalBinder.CheckImplicitThisCopyInReadOnlyMember(collectionExpr, getEnumeratorMethod, diagnostics);
+            }
             if (IsAsync)
             {
                 var placeholder = new BoundAwaitableValuePlaceholder(_syntax.Expression, builder.MoveNextMethod?.ReturnType ?? CreateErrorType());

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

@@ -19,7 +19,7 @@ namespace Microsoft.CodeAnalysis.CSharp {
     // class via a tool like ResGen or Visual Studio.
     // To add or remove a member, edit your .ResX file then rerun ResGen
     // with the /str option, or rebuild your VS project.
-    [global::System.CodeDom.Compiler.GeneratedCodeAttribute("System.Resources.Tools.StronglyTypedResourceBuilder", "15.0.0.0")]
+    [global::System.CodeDom.Compiler.GeneratedCodeAttribute("System.Resources.Tools.StronglyTypedResourceBuilder", "16.0.0.0")]
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]
     [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
     internal class CSharpResources {
@@ -14355,6 +14355,24 @@ internal class CSharpResources {
             }
         }
         
+        /// <summary>
+        ///   Looks up a localized string similar to Call to non-readonly member &apos;{0}&apos; from a &apos;readonly&apos; member results in an implicit copy of &apos;{1}&apos;..
+        /// </summary>
+        internal static string WRN_ImplicitCopyInReadOnlyMember {
+            get {
+                return ResourceManager.GetString("WRN_ImplicitCopyInReadOnlyMember", resourceCulture);
+            }
+        }
+        
+        /// <summary>
+        ///   Looks up a localized string similar to Call to non-readonly member from a &apos;readonly&apos; member results in an implicit copy..
+        /// </summary>
+        internal static string WRN_ImplicitCopyInReadOnlyMember_Title {
+            get {
+                return ResourceManager.GetString("WRN_ImplicitCopyInReadOnlyMember_Title", resourceCulture);
+            }
+        }
+        
         /// <summary>
         ///   Looks up a localized string similar to Assignment in conditional expression is always constant; did you mean to use == instead of = ?.
         /// </summary>

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -5421,6 +5421,12 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="WRN_NullLiteralMayIntroduceNullT_Title" xml:space="preserve">
     <value>A null literal introduces a null value for a type parameter.</value>
   </data>
+  <data name="WRN_ImplicitCopyInReadOnlyMember" xml:space="preserve">
+    <value>Call to non-readonly member '{0}' from a 'readonly' member results in an implicit copy of '{1}'.</value>
+  </data>
+  <data name="WRN_ImplicitCopyInReadOnlyMember_Title" xml:space="preserve">
+    <value>Call to non-readonly member from a 'readonly' member results in an implicit copy.</value>
+  </data>
   <data name="WRN_NullabilityMismatchInArgument" xml:space="preserve">
     <value>Argument of type '{0}' cannot be used for parameter '{2}' of type '{1}' in '{3}' due to differences in the nullability of reference types.</value>
   </data>

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

@@ -1683,6 +1683,7 @@ internal enum ErrorCode
         ERR_FeatureInPreview = 8652,
         WRN_DefaultExpressionMayIntroduceNullT = 8653,
         WRN_NullLiteralMayIntroduceNullT = 8654,
+        WRN_ImplicitCopyInReadOnlyMember = 8655,
 
         #endregion diagnostics introduced for C# 8.0
 

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

@@ -404,6 +404,7 @@ internal static int GetWarningLevel(ErrorCode code)
                 case ErrorCode.WRN_NullLiteralMayIntroduceNullT:
                 case ErrorCode.WRN_ConditionalAccessMayReturnNull:
                 case ErrorCode.WRN_AsOperatorMayReturnNull:
+                case ErrorCode.WRN_ImplicitCopyInReadOnlyMember:
                     return 1;
                 default:
                     return 0;

src/Compilers/CSharp/Portable/Generated/ErrorFacts.Generated.cs

@@ -222,6 +222,7 @@ public static bool IsWarning(ErrorCode code)
                 case ErrorCode.WRN_DuplicateInterfaceWithNullabilityMismatchInBaseList:
                 case ErrorCode.WRN_DefaultExpressionMayIntroduceNullT:
                 case ErrorCode.WRN_NullLiteralMayIntroduceNullT:
+                case ErrorCode.WRN_ImplicitCopyInReadOnlyMember:
                     return true;
                 default:
                     return false;

src/Compilers/CSharp/Portable/Symbols/MethodSymbol.cs

@@ -315,7 +315,7 @@ internal virtual bool IsExplicitInterfaceImplementation
         /// Indicates whether the method is effectively readonly,
         /// by either the method or the containing type being marked readonly.
         /// </summary>
-        internal bool IsEffectivelyReadOnly => IsDeclaredReadOnly || ContainingType?.IsReadOnly == true;
+        internal bool IsEffectivelyReadOnly => (IsDeclaredReadOnly || ContainingType?.IsReadOnly == true) && MethodKind != MethodKind.Constructor;
 
         /// <summary>
         /// Returns interface methods explicitly implemented by this method.

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

@@ -1262,6 +1262,16 @@
         <target state="translated">Po #pragma warning safeonly byl očekáván typ s možnou hodnotou null.</target>
         <note />
       </trans-unit>
+      <trans-unit id="WRN_ImplicitCopyInReadOnlyMember">
+        <source>Call to non-readonly member '{0}' from a 'readonly' member results in an implicit copy of '{1}'.</source>
+        <target state="new">Call to non-readonly member '{0}' from a 'readonly' member results in an implicit copy of '{1}'.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_ImplicitCopyInReadOnlyMember_Title">
+        <source>Call to non-readonly member from a 'readonly' member results in an implicit copy.</source>
+        <target state="new">Call to non-readonly member from a 'readonly' member results in an implicit copy.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="WRN_IsTypeNamedUnderscore">
         <source>The name '_' refers to the type '{0}', not the discard pattern. Use '@_' for the type, or 'var _' to discard.</source>
         <target state="translated">Název „_“ odkazuje na typ {0}, ne vzor discard. Použijte „@_“ pro tento typ nebo „var _“ pro zahození.</target>

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

@@ -1262,6 +1262,16 @@
         <target state="translated">Nach "#pragma warning safeonly" Nullable erwartet</target>
         <note />
       </trans-unit>
+      <trans-unit id="WRN_ImplicitCopyInReadOnlyMember">
+        <source>Call to non-readonly member '{0}' from a 'readonly' member results in an implicit copy of '{1}'.</source>
+        <target state="new">Call to non-readonly member '{0}' from a 'readonly' member results in an implicit copy of '{1}'.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_ImplicitCopyInReadOnlyMember_Title">
+        <source>Call to non-readonly member from a 'readonly' member results in an implicit copy.</source>
+        <target state="new">Call to non-readonly member from a 'readonly' member results in an implicit copy.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="WRN_IsTypeNamedUnderscore">
         <source>The name '_' refers to the type '{0}', not the discard pattern. Use '@_' for the type, or 'var _' to discard.</source>
         <target state="translated">Der Name "_" verweist auf den Typ "{0}", nicht auf das discard-Muster. Verwenden Sie "@_" für den Typ oder "var _" zum Verwerfen.</target>

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

@@ -1264,6 +1264,16 @@
         <target state="translated">Tipo que acepta valores NULL esperado después de advertencia de #pragma "safeonly"</target>
         <note />
       </trans-unit>
+      <trans-unit id="WRN_ImplicitCopyInReadOnlyMember">
+        <source>Call to non-readonly member '{0}' from a 'readonly' member results in an implicit copy of '{1}'.</source>
+        <target state="new">Call to non-readonly member '{0}' from a 'readonly' member results in an implicit copy of '{1}'.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_ImplicitCopyInReadOnlyMember_Title">
+        <source>Call to non-readonly member from a 'readonly' member results in an implicit copy.</source>
+        <target state="new">Call to non-readonly member from a 'readonly' member results in an implicit copy.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="WRN_IsTypeNamedUnderscore">
         <source>The name '_' refers to the type '{0}', not the discard pattern. Use '@_' for the type, or 'var _' to discard.</source>
         <target state="translated">El nombre "_" hace referencia al tipo "{0}", no al patrón de descarte. Use "@_" para el tipo o "var _" para el descarte.</target>

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

@@ -1263,6 +1263,16 @@
         <target state="translated">Nullable attendu après un avertissement #pragma safeonly</target>
         <note />
       </trans-unit>
+      <trans-unit id="WRN_ImplicitCopyInReadOnlyMember">
+        <source>Call to non-readonly member '{0}' from a 'readonly' member results in an implicit copy of '{1}'.</source>
+        <target state="new">Call to non-readonly member '{0}' from a 'readonly' member results in an implicit copy of '{1}'.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_ImplicitCopyInReadOnlyMember_Title">
+        <source>Call to non-readonly member from a 'readonly' member results in an implicit copy.</source>
+        <target state="new">Call to non-readonly member from a 'readonly' member results in an implicit copy.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="WRN_IsTypeNamedUnderscore">
         <source>The name '_' refers to the type '{0}', not the discard pattern. Use '@_' for the type, or 'var _' to discard.</source>
         <target state="translated">Le nom '_' fait référence au type '{0}', pas au modèle d'abandon. Utilisez '@_' pour le type, ou 'var _' pour abandonner.</target>

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

@@ -1262,6 +1262,16 @@
         <target state="translated">Dopo l'avviso della direttiva #pragma safeonly è previsto nullable</target>
         <note />
       </trans-unit>
+      <trans-unit id="WRN_ImplicitCopyInReadOnlyMember">
+        <source>Call to non-readonly member '{0}' from a 'readonly' member results in an implicit copy of '{1}'.</source>
+        <target state="new">Call to non-readonly member '{0}' from a 'readonly' member results in an implicit copy of '{1}'.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_ImplicitCopyInReadOnlyMember_Title">
+        <source>Call to non-readonly member from a 'readonly' member results in an implicit copy.</source>
+        <target state="new">Call to non-readonly member from a 'readonly' member results in an implicit copy.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="WRN_IsTypeNamedUnderscore">
         <source>The name '_' refers to the type '{0}', not the discard pattern. Use '@_' for the type, or 'var _' to discard.</source>
         <target state="translated">Il nome '_' fa riferimento al tipo '{0}' e non al criterio di eliminazione. Usare '@_' per il tipo oppure 'var _' per eliminare.</target>

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

@@ -1262,6 +1262,16 @@
         <target state="translated">#pragma warning safeonly の後に Null 許容が必要です</target>
         <note />
       </trans-unit>
+      <trans-unit id="WRN_ImplicitCopyInReadOnlyMember">
+        <source>Call to non-readonly member '{0}' from a 'readonly' member results in an implicit copy of '{1}'.</source>
+        <target state="new">Call to non-readonly member '{0}' from a 'readonly' member results in an implicit copy of '{1}'.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_ImplicitCopyInReadOnlyMember_Title">
+        <source>Call to non-readonly member from a 'readonly' member results in an implicit copy.</source>
+        <target state="new">Call to non-readonly member from a 'readonly' member results in an implicit copy.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="WRN_IsTypeNamedUnderscore">
         <source>The name '_' refers to the type '{0}', not the discard pattern. Use '@_' for the type, or 'var _' to discard.</source>
         <target state="translated">名前 '_' は、破棄パターンではなく型 '{0}' を参照しています。型の場合は '@_' を、破棄する場合は 'var _' をご使用ください。</target>

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

@@ -1262,6 +1262,16 @@
         <target state="translated">#pragma warning safeonly 뒤에 nullable이 필요합니다.</target>
         <note />
       </trans-unit>
+      <trans-unit id="WRN_ImplicitCopyInReadOnlyMember">
+        <source>Call to non-readonly member '{0}' from a 'readonly' member results in an implicit copy of '{1}'.</source>
+        <target state="new">Call to non-readonly member '{0}' from a 'readonly' member results in an implicit copy of '{1}'.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_ImplicitCopyInReadOnlyMember_Title">
+        <source>Call to non-readonly member from a 'readonly' member results in an implicit copy.</source>
+        <target state="new">Call to non-readonly member from a 'readonly' member results in an implicit copy.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="WRN_IsTypeNamedUnderscore">
         <source>The name '_' refers to the type '{0}', not the discard pattern. Use '@_' for the type, or 'var _' to discard.</source>
         <target state="translated">'_' 이름은 '{0}' 형식을 참조하며, 무시 패턴은 참조하지 않습니다. 형식에 '@_'을 사용하거나, 무시하려면 'var _'을 사용하세요.</target>

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

@@ -1262,6 +1262,16 @@
         <target state="translated">Oczekiwano elementu dopuszczającego wartość null po dyrektywne #pragma warning safeonly</target>
         <note />
       </trans-unit>
+      <trans-unit id="WRN_ImplicitCopyInReadOnlyMember">
+        <source>Call to non-readonly member '{0}' from a 'readonly' member results in an implicit copy of '{1}'.</source>
+        <target state="new">Call to non-readonly member '{0}' from a 'readonly' member results in an implicit copy of '{1}'.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_ImplicitCopyInReadOnlyMember_Title">
+        <source>Call to non-readonly member from a 'readonly' member results in an implicit copy.</source>
+        <target state="new">Call to non-readonly member from a 'readonly' member results in an implicit copy.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="WRN_IsTypeNamedUnderscore">
         <source>The name '_' refers to the type '{0}', not the discard pattern. Use '@_' for the type, or 'var _' to discard.</source>
         <target state="translated">Nazwa „_” odwołuje się do typu „{0}”, a nie do wzorca odrzucania. Użyj elementu „@_” aby odwołać się do typu, lub użyj elementu „var _”, aby odrzucić wartość.</target>

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

@@ -1262,6 +1262,16 @@
         <target state="translated">Esperado anulável após #pragma warning safeonly</target>
         <note />
       </trans-unit>
+      <trans-unit id="WRN_ImplicitCopyInReadOnlyMember">
+        <source>Call to non-readonly member '{0}' from a 'readonly' member results in an implicit copy of '{1}'.</source>
+        <target state="new">Call to non-readonly member '{0}' from a 'readonly' member results in an implicit copy of '{1}'.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_ImplicitCopyInReadOnlyMember_Title">
+        <source>Call to non-readonly member from a 'readonly' member results in an implicit copy.</source>
+        <target state="new">Call to non-readonly member from a 'readonly' member results in an implicit copy.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="WRN_IsTypeNamedUnderscore">
         <source>The name '_' refers to the type '{0}', not the discard pattern. Use '@_' for the type, or 'var _' to discard.</source>
         <target state="translated">O nome '_' refere-se ao tipo '{0}', não ao padrão de descarte. Use '@_' para o tipo ou 'var _' para descarte.</target>

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

@@ -1262,6 +1262,16 @@
         <target state="translated">После #pragma warning safeonly ожидается тип, допускающий значения NULL.</target>
         <note />
       </trans-unit>
+      <trans-unit id="WRN_ImplicitCopyInReadOnlyMember">
+        <source>Call to non-readonly member '{0}' from a 'readonly' member results in an implicit copy of '{1}'.</source>
+        <target state="new">Call to non-readonly member '{0}' from a 'readonly' member results in an implicit copy of '{1}'.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_ImplicitCopyInReadOnlyMember_Title">
+        <source>Call to non-readonly member from a 'readonly' member results in an implicit copy.</source>
+        <target state="new">Call to non-readonly member from a 'readonly' member results in an implicit copy.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="WRN_IsTypeNamedUnderscore">
         <source>The name '_' refers to the type '{0}', not the discard pattern. Use '@_' for the type, or 'var _' to discard.</source>
         <target state="translated">Имя "_" ссылается на тип "{0}", а не на шаблон отмены. Используйте "@_" в качестве типа или "var _" для отмены.</target>

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

@@ -1263,6 +1263,16 @@
         <target state="translated">#pragma uyarısı safeonly sonrasında boş değer atanabilir beklendi</target>
         <note />
       </trans-unit>
+      <trans-unit id="WRN_ImplicitCopyInReadOnlyMember">
+        <source>Call to non-readonly member '{0}' from a 'readonly' member results in an implicit copy of '{1}'.</source>
+        <target state="new">Call to non-readonly member '{0}' from a 'readonly' member results in an implicit copy of '{1}'.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_ImplicitCopyInReadOnlyMember_Title">
+        <source>Call to non-readonly member from a 'readonly' member results in an implicit copy.</source>
+        <target state="new">Call to non-readonly member from a 'readonly' member results in an implicit copy.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="WRN_IsTypeNamedUnderscore">
         <source>The name '_' refers to the type '{0}', not the discard pattern. Use '@_' for the type, or 'var _' to discard.</source>
         <target state="translated">'_' adı atma desenine değil '{0}' türüne başvuruyor. Tür için '@_' adını veya atmak için 'var _' adını kullanın.</target>

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

@@ -1263,6 +1263,16 @@
         <target state="translated">在 #pragma warning safeonly 之后应是可为 null</target>
         <note />
       </trans-unit>
+      <trans-unit id="WRN_ImplicitCopyInReadOnlyMember">
+        <source>Call to non-readonly member '{0}' from a 'readonly' member results in an implicit copy of '{1}'.</source>
+        <target state="new">Call to non-readonly member '{0}' from a 'readonly' member results in an implicit copy of '{1}'.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_ImplicitCopyInReadOnlyMember_Title">
+        <source>Call to non-readonly member from a 'readonly' member results in an implicit copy.</source>
+        <target state="new">Call to non-readonly member from a 'readonly' member results in an implicit copy.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="WRN_IsTypeNamedUnderscore">
         <source>The name '_' refers to the type '{0}', not the discard pattern. Use '@_' for the type, or 'var _' to discard.</source>
         <target state="translated">名称 "_" 引用类型“{0}”，而不引用放弃模式。对于类型，请使用 "@_"；对于弃用，请使用 "var _"。</target>

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

@@ -1262,6 +1262,16 @@
         <target state="translated">在#pragma warning safeonly 後面必須是可為 Null 的項目</target>
         <note />
       </trans-unit>
+      <trans-unit id="WRN_ImplicitCopyInReadOnlyMember">
+        <source>Call to non-readonly member '{0}' from a 'readonly' member results in an implicit copy of '{1}'.</source>
+        <target state="new">Call to non-readonly member '{0}' from a 'readonly' member results in an implicit copy of '{1}'.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_ImplicitCopyInReadOnlyMember_Title">
+        <source>Call to non-readonly member from a 'readonly' member results in an implicit copy.</source>
+        <target state="new">Call to non-readonly member from a 'readonly' member results in an implicit copy.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="WRN_IsTypeNamedUnderscore">
         <source>The name '_' refers to the type '{0}', not the discard pattern. Use '@_' for the type, or 'var _' to discard.</source>
         <target state="translated">名稱 '_' 參考類型 '{0}'，而非捨棄模式。請為類型使用 '@_'，或使用 'var _' 捨棄。</target>

src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenReadonlyStructTests.cs

@@ -1342,8 +1342,12 @@ static void Main()
 ";
 
             var verifier = CompileAndVerify(csharp, expectedOutput: "123");
-            // PROTOTYPE: should warn about copying 'this' when calling M2
-            verifier.VerifyDiagnostics();
+
+            verifier.VerifyDiagnostics(
+                // (9,9): warning CS8655: Call to non-readonly member 'M2' from a 'readonly' member results in an implicit copy of 'this'.
+                //         M2();
+                Diagnostic(ErrorCode.WRN_ImplicitCopyInReadOnlyMember, "M2").WithArguments("M2", "this").WithLocation(9, 9));
+
             verifier.VerifyIL("S.M1", @"
 {
   // Code size       51 (0x33)
@@ -1404,7 +1408,7 @@ static void Main()
 ";
 
             var verifier = CompileAndVerify(csharp, expectedOutput: "123");
-            // PROTOTYPE: should warn about copying 's' when calling M2
+            // should warn about calling s.M2 in warning wave (see https://github.com/dotnet/roslyn/issues/33968)
             verifier.VerifyDiagnostics();
             verifier.VerifyIL("S.M1", @"
 {

src/Compilers/CSharp/Test/Syntax/Diagnostics/DiagnosticTest.cs

@@ -301,6 +301,7 @@ public void WarningLevel_2()
                         case ErrorCode.WRN_AsOperatorMayReturnNull:
                         case ErrorCode.WRN_DefaultExpressionMayIntroduceNullT:
                         case ErrorCode.WRN_NullLiteralMayIntroduceNullT:
+                        case ErrorCode.WRN_ImplicitCopyInReadOnlyMember:
                             Assert.Equal(1, ErrorFacts.GetWarningLevel(errorCode));
                             break;
                         case ErrorCode.WRN_InvalidVersionFormat: