Readonly members more test coverage (#34602)

* Check parser behavior for bad expression property declaration

* Make readonly modifiers more strict on accessors to match accessibility behavior

* Fix tests

* Improve implicit copy warning message. Add compound property assignment test.

* Test ErrorCode.ERR_ReadOnlyModMissingAccessor with an indexer

* Simplify error message for 'static readonly' members

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

@@ -1153,7 +1153,7 @@ internal bool CheckImplicitThisCopyInReadOnlyMember(BoundExpression receiver, Me
                 !method.IsEffectivelyReadOnly &&
                 !method.IsStatic)
             {
-                Error(diagnostics, ErrorCode.WRN_ImplicitCopyInReadOnlyMember, receiver.Syntax, method.Name, ThisParameterSymbol.SymbolName);
+                Error(diagnostics, ErrorCode.WRN_ImplicitCopyInReadOnlyMember, receiver.Syntax, method, ThisParameterSymbol.SymbolName);
                 return false;
             }
 

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

@@ -8763,6 +8763,15 @@ internal class CSharpResources {
             }
         }
         
+        /// <summary>
+        ///   Looks up a localized string similar to &apos;{0}&apos;: &apos;readonly&apos; can only be used on accessors if the property or indexer has both a get and a set accessor.
+        /// </summary>
+        internal static string ERR_ReadOnlyModMissingAccessor {
+            get {
+                return ResourceManager.GetString("ERR_ReadOnlyModMissingAccessor", resourceCulture);
+            }
+        }
+        
         /// <summary>
         ///   Looks up a localized string similar to Members of readonly field &apos;{0}&apos; of type &apos;{1}&apos; cannot be assigned with an object initializer because it is of a value type.
         /// </summary>
@@ -9664,7 +9673,7 @@ internal class CSharpResources {
         }
         
         /// <summary>
-        ///   Looks up a localized string similar to Static member &apos;{0}&apos; cannot be marked &apos;readonly&apos; because readonly members cannot modify &apos;this&apos; and static members do not have a &apos;this&apos; parameter..
+        ///   Looks up a localized string similar to Static member &apos;{0}&apos; cannot be marked &apos;readonly&apos;..
         /// </summary>
         internal static string ERR_StaticMemberCantBeReadOnly {
             get {

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -5431,7 +5431,7 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
     <value>Call to non-readonly member from a 'readonly' member results in an implicit copy.</value>
   </data>
   <data name="ERR_StaticMemberCantBeReadOnly" xml:space="preserve">
-    <value>Static member '{0}' cannot be marked 'readonly' because readonly members cannot modify 'this' and static members do not have a 'this' parameter.</value>
+    <value>Static member '{0}' cannot be marked 'readonly'.</value>
   </data>
   <data name="ERR_AutoSetterCantBeReadOnly" xml:space="preserve">
     <value>Auto-implemented 'set' accessor '{0}' cannot be marked 'readonly'.</value>
@@ -5451,6 +5451,9 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_PartialMethodReadOnlyDifference" xml:space="preserve">
     <value>Both partial method declarations must be readonly or neither may be readonly</value>
   </data>
+  <data name="ERR_ReadOnlyModMissingAccessor" xml:space="preserve">
+    <value>'{0}': 'readonly' can only be used on accessors if the property or indexer has both a get and a set accessor</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

@@ -1691,6 +1691,7 @@ internal enum ErrorCode
         ERR_DuplicatePropertyReadOnlyMods = 8660,
         ERR_FieldLikeEventCantBeReadOnly = 8661,
         ERR_PartialMethodReadOnlyDifference = 8662,
+        ERR_ReadOnlyModMissingAccessor = 8663
 
         #endregion diagnostics introduced for C# 8.0
 

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

@@ -480,7 +480,7 @@ protected void CheckModifiersAndType(DiagnosticBag diagnostics)
             }
             else if (IsReadOnly && IsStatic)
             {
-                // Static member '{0}' cannot be marked 'readonly' because readonly members cannot modify 'this' and static members do not have a 'this' parameter.
+                // Static member '{0}' cannot be marked 'readonly'.
                 diagnostics.Add(ErrorCode.ERR_StaticMemberCantBeReadOnly, location, this);
             }
             else if (IsReadOnly && HasAssociatedField)

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

@@ -940,7 +940,7 @@ private void CheckModifiers(bool hasBody, Location location, DiagnosticBag diagn
             }
             else if (IsStatic && IsDeclaredReadOnly)
             {
-                // Static member '{0}' cannot be marked 'readonly' because readonly members cannot modify 'this' and static members do not have a 'this' parameter.
+                // Static member '{0}' cannot be marked 'readonly'.
                 diagnostics.Add(ErrorCode.ERR_StaticMemberCantBeReadOnly, location, this);
             }
             else if (IsAbstract && !ContainingType.IsAbstract && (ContainingType.TypeKind == TypeKind.Class || ContainingType.TypeKind == TypeKind.Submission))

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

@@ -477,7 +477,7 @@ private void CheckModifiers(Location location, bool hasBody, bool isAutoProperty
             }
             else if (LocalDeclaredReadOnly && IsStatic)
             {
-                // Static member '{0}' cannot be marked 'readonly' because readonly members cannot modify 'this' and static members do not have a 'this' parameter.
+                // Static member '{0}' cannot be marked 'readonly'.
                 diagnostics.Add(ErrorCode.ERR_StaticMemberCantBeReadOnly, location, this);
             }
             else if (LocalDeclaredReadOnly && _isAutoPropertyAccessor && MethodKind == MethodKind.PropertySet)

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

@@ -385,6 +385,12 @@ internal sealed class SourcePropertySymbol : PropertySymbol, IAttributeTargetSym
                             {
                                 diagnostics.Add(ErrorCode.ERR_AccessModMissingAccessor, location, this);
                             }
+
+                            // Check that 'readonly' is not set on the one accessor.
+                            if (accessor.LocalDeclaredReadOnly)
+                            {
+                                diagnostics.Add(ErrorCode.ERR_ReadOnlyModMissingAccessor, location, this);
+                            }
                         }
                     }
                 }
@@ -910,7 +916,7 @@ private void CheckModifiers(Location location, bool isIndexer, DiagnosticBag dia
             }
             else if (IsStatic && HasReadOnlyModifier)
             {
-                // Static member '{0}' cannot be marked 'readonly' because readonly members cannot modify 'this' and static members do not have a 'this' parameter.
+                // Static member '{0}' cannot be marked 'readonly'.
                 diagnostics.Add(ErrorCode.ERR_StaticMemberCantBeReadOnly, location, this);
             }
             else if (IsOverride && (IsNew || IsVirtual))

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

@@ -292,6 +292,11 @@
         <target state="translated">Dílčí vzor vlastnosti vyžaduje odkaz na vlastnost nebo pole k přiřazení, např. „{{ Name: {0} }}“.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ReadOnlyModMissingAccessor">
+        <source>'{0}': 'readonly' can only be used on accessors if the property or indexer has both a get and a set accessor</source>
+        <target state="new">'{0}': 'readonly' can only be used on accessors if the property or indexer has both a get and a set accessor</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_RefAssignNarrower">
         <source>Cannot ref-assign '{1}' to '{0}' because '{1}' has a narrower escape scope than '{0}'.</source>
         <target state="translated">Přiřazení odkazu {1} k {0} nelze provést, protože {1} má užší řídicí obor než {0}.</target>
@@ -318,8 +323,8 @@
         <note />
       </trans-unit>
       <trans-unit id="ERR_StaticMemberCantBeReadOnly">
-        <source>Static member '{0}' cannot be marked 'readonly' because readonly members cannot modify 'this' and static members do not have a 'this' parameter.</source>
-        <target state="new">Static member '{0}' cannot be marked 'readonly' because readonly members cannot modify 'this' and static members do not have a 'this' parameter.</target>
+        <source>Static member '{0}' cannot be marked 'readonly'.</source>
+        <target state="new">Static member '{0}' cannot be marked 'readonly'.</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_SwitchArmSubsumed">

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

@@ -292,6 +292,11 @@
         <target state="translated">Ein Eigenschaftsteilmuster erfordert einen Verweis auf die abzugleichende Eigenschaft oder das abzugleichende Feld. Beispiel: "{{ Name: {0} }}"</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ReadOnlyModMissingAccessor">
+        <source>'{0}': 'readonly' can only be used on accessors if the property or indexer has both a get and a set accessor</source>
+        <target state="new">'{0}': 'readonly' can only be used on accessors if the property or indexer has both a get and a set accessor</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_RefAssignNarrower">
         <source>Cannot ref-assign '{1}' to '{0}' because '{1}' has a narrower escape scope than '{0}'.</source>
         <target state="translated">ref-assign von "{1}" zu "{0}" ist nicht möglich, weil "{1}" einen geringeren Escapebereich als "{0}" aufweist.</target>
@@ -318,8 +323,8 @@
         <note />
       </trans-unit>
       <trans-unit id="ERR_StaticMemberCantBeReadOnly">
-        <source>Static member '{0}' cannot be marked 'readonly' because readonly members cannot modify 'this' and static members do not have a 'this' parameter.</source>
-        <target state="new">Static member '{0}' cannot be marked 'readonly' because readonly members cannot modify 'this' and static members do not have a 'this' parameter.</target>
+        <source>Static member '{0}' cannot be marked 'readonly'.</source>
+        <target state="new">Static member '{0}' cannot be marked 'readonly'.</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_SwitchArmSubsumed">

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

@@ -292,6 +292,11 @@
         <target state="translated">El subpatrón de una propiedad requiere una referencia a la propiedad o al campo que debe coincidir; por ejemplo, "{{ Name: {0} }}"</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ReadOnlyModMissingAccessor">
+        <source>'{0}': 'readonly' can only be used on accessors if the property or indexer has both a get and a set accessor</source>
+        <target state="new">'{0}': 'readonly' can only be used on accessors if the property or indexer has both a get and a set accessor</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_RefAssignNarrower">
         <source>Cannot ref-assign '{1}' to '{0}' because '{1}' has a narrower escape scope than '{0}'.</source>
         <target state="translated">No se puede asignar referencia "{1}" a "{0}" porque "{1}" tiene un ámbito de escape más limitado que "{0}".</target>
@@ -318,8 +323,8 @@
         <note />
       </trans-unit>
       <trans-unit id="ERR_StaticMemberCantBeReadOnly">
-        <source>Static member '{0}' cannot be marked 'readonly' because readonly members cannot modify 'this' and static members do not have a 'this' parameter.</source>
-        <target state="new">Static member '{0}' cannot be marked 'readonly' because readonly members cannot modify 'this' and static members do not have a 'this' parameter.</target>
+        <source>Static member '{0}' cannot be marked 'readonly'.</source>
+        <target state="new">Static member '{0}' cannot be marked 'readonly'.</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_SwitchArmSubsumed">

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

@@ -292,6 +292,11 @@
         <target state="translated">Un sous-modèle de propriété nécessite une correspondance de la référence à la propriété ou au champ. Exemple : '{{ Nom: {0} }}'</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ReadOnlyModMissingAccessor">
+        <source>'{0}': 'readonly' can only be used on accessors if the property or indexer has both a get and a set accessor</source>
+        <target state="new">'{0}': 'readonly' can only be used on accessors if the property or indexer has both a get and a set accessor</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_RefAssignNarrower">
         <source>Cannot ref-assign '{1}' to '{0}' because '{1}' has a narrower escape scope than '{0}'.</source>
         <target state="translated">Impossible d'effectuer une assignation par référence de '{1}' vers '{0}', car '{1}' a une portée de sortie plus limitée que '{0}'.</target>
@@ -318,8 +323,8 @@
         <note />
       </trans-unit>
       <trans-unit id="ERR_StaticMemberCantBeReadOnly">
-        <source>Static member '{0}' cannot be marked 'readonly' because readonly members cannot modify 'this' and static members do not have a 'this' parameter.</source>
-        <target state="new">Static member '{0}' cannot be marked 'readonly' because readonly members cannot modify 'this' and static members do not have a 'this' parameter.</target>
+        <source>Static member '{0}' cannot be marked 'readonly'.</source>
+        <target state="new">Static member '{0}' cannot be marked 'readonly'.</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_SwitchArmSubsumed">

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

@@ -292,6 +292,11 @@
         <target state="translated">Con un criterio secondario di proprietà è richiesto un riferimento alla proprietà o al campo da abbinare, ad esempio '{{ Name: {0} }}'</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ReadOnlyModMissingAccessor">
+        <source>'{0}': 'readonly' can only be used on accessors if the property or indexer has both a get and a set accessor</source>
+        <target state="new">'{0}': 'readonly' can only be used on accessors if the property or indexer has both a get and a set accessor</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_RefAssignNarrower">
         <source>Cannot ref-assign '{1}' to '{0}' because '{1}' has a narrower escape scope than '{0}'.</source>
         <target state="translated">Non è possibile assegnare '{1}' a '{0}' come ref perché l'ambito di escape di '{1}' è ridotto rispetto a quello di '{0}'.</target>
@@ -318,8 +323,8 @@
         <note />
       </trans-unit>
       <trans-unit id="ERR_StaticMemberCantBeReadOnly">
-        <source>Static member '{0}' cannot be marked 'readonly' because readonly members cannot modify 'this' and static members do not have a 'this' parameter.</source>
-        <target state="new">Static member '{0}' cannot be marked 'readonly' because readonly members cannot modify 'this' and static members do not have a 'this' parameter.</target>
+        <source>Static member '{0}' cannot be marked 'readonly'.</source>
+        <target state="new">Static member '{0}' cannot be marked 'readonly'.</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_SwitchArmSubsumed">

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

@@ -292,6 +292,11 @@
         <target state="translated">プロパティ サブパターンには、一致させるプロパティまたはフィールドへの参照が必要です。例: '{{ Name: {0} }}'</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ReadOnlyModMissingAccessor">
+        <source>'{0}': 'readonly' can only be used on accessors if the property or indexer has both a get and a set accessor</source>
+        <target state="new">'{0}': 'readonly' can only be used on accessors if the property or indexer has both a get and a set accessor</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_RefAssignNarrower">
         <source>Cannot ref-assign '{1}' to '{0}' because '{1}' has a narrower escape scope than '{0}'.</source>
         <target state="translated">'{1}' を '{0}' に ref 割り当てすることはできません。'{1}' のエスケープ スコープが '{0}' より狭いためです。</target>
@@ -318,8 +323,8 @@
         <note />
       </trans-unit>
       <trans-unit id="ERR_StaticMemberCantBeReadOnly">
-        <source>Static member '{0}' cannot be marked 'readonly' because readonly members cannot modify 'this' and static members do not have a 'this' parameter.</source>
-        <target state="new">Static member '{0}' cannot be marked 'readonly' because readonly members cannot modify 'this' and static members do not have a 'this' parameter.</target>
+        <source>Static member '{0}' cannot be marked 'readonly'.</source>
+        <target state="new">Static member '{0}' cannot be marked 'readonly'.</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_SwitchArmSubsumed">

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

@@ -292,6 +292,11 @@
         <target state="translated">속성 하위 패턴은 일치시킬 속성 또는 필드에 대한 참조가 필요합니다(예: '{{ Name: {0} }}')</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ReadOnlyModMissingAccessor">
+        <source>'{0}': 'readonly' can only be used on accessors if the property or indexer has both a get and a set accessor</source>
+        <target state="new">'{0}': 'readonly' can only be used on accessors if the property or indexer has both a get and a set accessor</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_RefAssignNarrower">
         <source>Cannot ref-assign '{1}' to '{0}' because '{1}' has a narrower escape scope than '{0}'.</source>
         <target state="translated">'{1}'을(를) '{0}'에 참조 할당할 수 없습니다. '{1}'이(가) '{0}'보다 이스케이프 범위가 좁기 때문입니다.</target>
@@ -318,8 +323,8 @@
         <note />
       </trans-unit>
       <trans-unit id="ERR_StaticMemberCantBeReadOnly">
-        <source>Static member '{0}' cannot be marked 'readonly' because readonly members cannot modify 'this' and static members do not have a 'this' parameter.</source>
-        <target state="new">Static member '{0}' cannot be marked 'readonly' because readonly members cannot modify 'this' and static members do not have a 'this' parameter.</target>
+        <source>Static member '{0}' cannot be marked 'readonly'.</source>
+        <target state="new">Static member '{0}' cannot be marked 'readonly'.</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_SwitchArmSubsumed">

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

@@ -292,6 +292,11 @@
         <target state="translated">Wzorzec podrzędny właściwości wymaga odwołania do właściwości lub pola, które należy dopasować, na przykład „{{ Name: {0} }}”</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ReadOnlyModMissingAccessor">
+        <source>'{0}': 'readonly' can only be used on accessors if the property or indexer has both a get and a set accessor</source>
+        <target state="new">'{0}': 'readonly' can only be used on accessors if the property or indexer has both a get and a set accessor</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_RefAssignNarrower">
         <source>Cannot ref-assign '{1}' to '{0}' because '{1}' has a narrower escape scope than '{0}'.</source>
         <target state="translated">Nie można przypisać odwołania elementu „{1}” do elementu „{0}”, ponieważ element „{1}” ma węższy zakres wyjścia niż element „{0}”.</target>
@@ -318,8 +323,8 @@
         <note />
       </trans-unit>
       <trans-unit id="ERR_StaticMemberCantBeReadOnly">
-        <source>Static member '{0}' cannot be marked 'readonly' because readonly members cannot modify 'this' and static members do not have a 'this' parameter.</source>
-        <target state="new">Static member '{0}' cannot be marked 'readonly' because readonly members cannot modify 'this' and static members do not have a 'this' parameter.</target>
+        <source>Static member '{0}' cannot be marked 'readonly'.</source>
+        <target state="new">Static member '{0}' cannot be marked 'readonly'.</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_SwitchArmSubsumed">

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

@@ -292,6 +292,11 @@
         <target state="translated">Um subpadrão de propriedade requer que uma referência à propriedade ou ao campo seja correspondida, por exemplo, '{{ Name: {0} }}'</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ReadOnlyModMissingAccessor">
+        <source>'{0}': 'readonly' can only be used on accessors if the property or indexer has both a get and a set accessor</source>
+        <target state="new">'{0}': 'readonly' can only be used on accessors if the property or indexer has both a get and a set accessor</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_RefAssignNarrower">
         <source>Cannot ref-assign '{1}' to '{0}' because '{1}' has a narrower escape scope than '{0}'.</source>
         <target state="translated">Não é possível atribuir ref '{1}' a '{0}' porque '{1}' tem um escopo de escape mais limitado que '{0}'.</target>
@@ -318,8 +323,8 @@
         <note />
       </trans-unit>
       <trans-unit id="ERR_StaticMemberCantBeReadOnly">
-        <source>Static member '{0}' cannot be marked 'readonly' because readonly members cannot modify 'this' and static members do not have a 'this' parameter.</source>
-        <target state="new">Static member '{0}' cannot be marked 'readonly' because readonly members cannot modify 'this' and static members do not have a 'this' parameter.</target>
+        <source>Static member '{0}' cannot be marked 'readonly'.</source>
+        <target state="new">Static member '{0}' cannot be marked 'readonly'.</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_SwitchArmSubsumed">

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

@@ -292,6 +292,11 @@
         <target state="translated">Для вложенного шаблона свойств требуется ссылка на свойство или поле для сопоставления, например, "{{ Name: {0} }}".</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ReadOnlyModMissingAccessor">
+        <source>'{0}': 'readonly' can only be used on accessors if the property or indexer has both a get and a set accessor</source>
+        <target state="new">'{0}': 'readonly' can only be used on accessors if the property or indexer has both a get and a set accessor</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_RefAssignNarrower">
         <source>Cannot ref-assign '{1}' to '{0}' because '{1}' has a narrower escape scope than '{0}'.</source>
         <target state="translated">Не удается присвоить по ссылке "{1}" для "{0}", так как escape-область у "{1}" уже, чем у "{0}".</target>
@@ -318,8 +323,8 @@
         <note />
       </trans-unit>
       <trans-unit id="ERR_StaticMemberCantBeReadOnly">
-        <source>Static member '{0}' cannot be marked 'readonly' because readonly members cannot modify 'this' and static members do not have a 'this' parameter.</source>
-        <target state="new">Static member '{0}' cannot be marked 'readonly' because readonly members cannot modify 'this' and static members do not have a 'this' parameter.</target>
+        <source>Static member '{0}' cannot be marked 'readonly'.</source>
+        <target state="new">Static member '{0}' cannot be marked 'readonly'.</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_SwitchArmSubsumed">

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

@@ -292,6 +292,11 @@
         <target state="translated">Bir özellik alt deseni, özellik veya alan başvurusunun eşleşmesini gerektiriyor, ör. '{{ Name: {0} }}'</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ReadOnlyModMissingAccessor">
+        <source>'{0}': 'readonly' can only be used on accessors if the property or indexer has both a get and a set accessor</source>
+        <target state="new">'{0}': 'readonly' can only be used on accessors if the property or indexer has both a get and a set accessor</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_RefAssignNarrower">
         <source>Cannot ref-assign '{1}' to '{0}' because '{1}' has a narrower escape scope than '{0}'.</source>
         <target state="translated">'{1}', '{0}' öğesinden daha dar bir kaçış kapsamı içerdiğinden '{0}' öğesine '{1}' ref ataması yapılamıyor.</target>
@@ -318,8 +323,8 @@
         <note />
       </trans-unit>
       <trans-unit id="ERR_StaticMemberCantBeReadOnly">
-        <source>Static member '{0}' cannot be marked 'readonly' because readonly members cannot modify 'this' and static members do not have a 'this' parameter.</source>
-        <target state="new">Static member '{0}' cannot be marked 'readonly' because readonly members cannot modify 'this' and static members do not have a 'this' parameter.</target>
+        <source>Static member '{0}' cannot be marked 'readonly'.</source>
+        <target state="new">Static member '{0}' cannot be marked 'readonly'.</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_SwitchArmSubsumed">

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

@@ -292,6 +292,11 @@
         <target state="translated">属性子模式需要引用要匹配的属性或字段，例如，"{{ Name: {0} }}"</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ReadOnlyModMissingAccessor">
+        <source>'{0}': 'readonly' can only be used on accessors if the property or indexer has both a get and a set accessor</source>
+        <target state="new">'{0}': 'readonly' can only be used on accessors if the property or indexer has both a get and a set accessor</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_RefAssignNarrower">
         <source>Cannot ref-assign '{1}' to '{0}' because '{1}' has a narrower escape scope than '{0}'.</source>
         <target state="translated">无法将“{1}”重新赋值为“{0}”，因为“{1}”具有比“{0}”更窄的转义范围。</target>
@@ -318,8 +323,8 @@
         <note />
       </trans-unit>
       <trans-unit id="ERR_StaticMemberCantBeReadOnly">
-        <source>Static member '{0}' cannot be marked 'readonly' because readonly members cannot modify 'this' and static members do not have a 'this' parameter.</source>
-        <target state="new">Static member '{0}' cannot be marked 'readonly' because readonly members cannot modify 'this' and static members do not have a 'this' parameter.</target>
+        <source>Static member '{0}' cannot be marked 'readonly'.</source>
+        <target state="new">Static member '{0}' cannot be marked 'readonly'.</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_SwitchArmSubsumed">

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

@@ -292,6 +292,11 @@
         <target state="translated">屬性子模式需要對屬性或欄位的參考才能比對，例如 '{{ Name: {0} }}'</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ReadOnlyModMissingAccessor">
+        <source>'{0}': 'readonly' can only be used on accessors if the property or indexer has both a get and a set accessor</source>
+        <target state="new">'{0}': 'readonly' can only be used on accessors if the property or indexer has both a get and a set accessor</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_RefAssignNarrower">
         <source>Cannot ref-assign '{1}' to '{0}' because '{1}' has a narrower escape scope than '{0}'.</source>
         <target state="translated">不能將 '{1}' 參考指派至 '{0}'，因為 '{1}' 的逸出範圍比 '{0}' 還要窄。</target>
@@ -318,8 +323,8 @@
         <note />
       </trans-unit>
       <trans-unit id="ERR_StaticMemberCantBeReadOnly">
-        <source>Static member '{0}' cannot be marked 'readonly' because readonly members cannot modify 'this' and static members do not have a 'this' parameter.</source>
-        <target state="new">Static member '{0}' cannot be marked 'readonly' because readonly members cannot modify 'this' and static members do not have a 'this' parameter.</target>
+        <source>Static member '{0}' cannot be marked 'readonly'.</source>
+        <target state="new">Static member '{0}' cannot be marked 'readonly'.</target>
         <note />
       </trans-unit>
       <trans-unit id="ERR_SwitchArmSubsumed">

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

@@ -1319,8 +1319,8 @@ public struct S
 
     public int P1 { get; set; }
     public readonly int P2 => 42;
-    public int P3 { readonly get => 123; }
-    public int P4 { readonly set {} }
+    public int P3 { readonly get => 123; set {} }
+    public int P4 { get => 123; readonly set {} }
     public static int P5 { get; set; }
 }
 ";
@@ -1358,8 +1358,12 @@ void validate(ModuleSymbol module)
                 Assert.False(peModule.Module.HasIsReadOnlyAttribute(((PEPropertySymbol)p3).Handle));
                 Assert.True(peModule.Module.HasIsReadOnlyAttribute(((PEMethodSymbol)p3.GetMethod).Handle));
                 Assert.False(peModule.Module.HasIsReadOnlyAttribute(((PEMethodSymbol)p3.GetMethod).Signature.ReturnParam.Handle));
+                Assert.False(peModule.Module.HasIsReadOnlyAttribute(((PEMethodSymbol)p3.SetMethod).Handle));
+                Assert.False(peModule.Module.HasIsReadOnlyAttribute(((PEMethodSymbol)p3.SetMethod).Signature.ReturnParam.Handle));
 
                 Assert.False(peModule.Module.HasIsReadOnlyAttribute(((PEPropertySymbol)p4).Handle));
+                Assert.False(peModule.Module.HasIsReadOnlyAttribute(((PEMethodSymbol)p4.GetMethod).Handle));
+                Assert.False(peModule.Module.HasIsReadOnlyAttribute(((PEMethodSymbol)p4.GetMethod).Signature.ReturnParam.Handle));
                 Assert.True(peModule.Module.HasIsReadOnlyAttribute(((PEMethodSymbol)p4.SetMethod).Handle));
                 Assert.False(peModule.Module.HasIsReadOnlyAttribute(((PEMethodSymbol)p4.SetMethod).Signature.ReturnParam.Handle));
 
@@ -1453,8 +1457,8 @@ public void ReadOnlyStruct_ReadOnlyMembers_Metadata()
 
     public int P1 { get; }
     public readonly int P2 => 42;
-    public int P3 { readonly get => 123; }
-    public int P4 { readonly set {} }
+    public int P3 { readonly get => 123; set {} }
+    public int P4 { get => 123; readonly set {} }
     public static int P5 { get; set; }
 }
 ";
@@ -1490,8 +1494,12 @@ void validate(ModuleSymbol module)
                 Assert.False(peModule.Module.HasIsReadOnlyAttribute(((PEPropertySymbol)p3).Handle));
                 Assert.True(peModule.Module.HasIsReadOnlyAttribute(((PEMethodSymbol)p3.GetMethod).Handle));
                 Assert.False(peModule.Module.HasIsReadOnlyAttribute(((PEMethodSymbol)p3.GetMethod).Signature.ReturnParam.Handle));
+                Assert.False(peModule.Module.HasIsReadOnlyAttribute(((PEMethodSymbol)p3.SetMethod).Handle));
+                Assert.False(peModule.Module.HasIsReadOnlyAttribute(((PEMethodSymbol)p3.SetMethod).Signature.ReturnParam.Handle));
 
                 Assert.False(peModule.Module.HasIsReadOnlyAttribute(((PEPropertySymbol)p4).Handle));
+                Assert.False(peModule.Module.HasIsReadOnlyAttribute(((PEMethodSymbol)p4.GetMethod).Handle));
+                Assert.False(peModule.Module.HasIsReadOnlyAttribute(((PEMethodSymbol)p4.GetMethod).Signature.ReturnParam.Handle));
                 Assert.True(peModule.Module.HasIsReadOnlyAttribute(((PEMethodSymbol)p4.SetMethod).Handle));
                 Assert.False(peModule.Module.HasIsReadOnlyAttribute(((PEMethodSymbol)p4.SetMethod).Signature.ReturnParam.Handle));
 
@@ -1518,8 +1526,8 @@ public struct S1
 
     public int P1 { get; set; }
     public readonly int P2 => 42;
-    public int P3 { readonly get => 123; }
-    public int P4 { readonly set {} }
+    public int P3 { readonly get => 123; set {} }
+    public int P4 { get => 123; readonly set {} }
     public static int P5 { get; set; }
     public readonly event Action<EventArgs> E { add {} remove {} }
 }
@@ -1535,6 +1543,7 @@ public struct S1
 }
 ";
             var externalComp = CreateCompilation(external);
+            externalComp.VerifyDiagnostics();
             verify(externalComp);
 
             var comp = CreateCompilation("", references: new[] { externalComp.EmitToImageReference() });
@@ -1554,7 +1563,11 @@ void verify(CSharpCompilation comp)
                 verifyReadOnly(s1.GetProperty("P1").SetMethod, false, false, RefKind.Ref);
 
                 verifyReadOnly(s1.GetProperty("P2").GetMethod, true, true, RefKind.RefReadOnly);
+
                 verifyReadOnly(s1.GetProperty("P3").GetMethod, true, true, RefKind.RefReadOnly);
+                verifyReadOnly(s1.GetProperty("P3").SetMethod, false, false, RefKind.Ref);
+
+                verifyReadOnly(s1.GetProperty("P4").GetMethod, false, false, RefKind.Ref);
                 verifyReadOnly(s1.GetProperty("P4").SetMethod, true, true, RefKind.RefReadOnly);
 
                 verifyReadOnly(s1.GetProperty("P5").GetMethod, false, false, null);
@@ -1696,9 +1709,9 @@ static void Main()
             var verifier = CompileAndVerify(csharp, expectedOutput: "123");
 
             verifier.VerifyDiagnostics(
-                // (9,9): warning CS8655: Call to non-readonly member 'M2' from a 'readonly' member results in an implicit copy of 'this'.
+                // (9,9): warning CS8655: Call to non-readonly member 'S.M2()' from a 'readonly' member results in an implicit copy of 'this'.
                 //         M2();
-                Diagnostic(ErrorCode.WRN_ImplicitCopyInReadOnlyMember, "M2").WithArguments("M2", "this").WithLocation(9, 9));
+                Diagnostic(ErrorCode.WRN_ImplicitCopyInReadOnlyMember, "M2").WithArguments("S.M2()", "this").WithLocation(9, 9));
 
             verifier.VerifyIL("S.M1", @"
 {
@@ -2195,15 +2208,15 @@ public struct S
             var verifier = CompileAndVerify(csharp);
 
             verifier.VerifyDiagnostics(
-                // (12,9): warning CS8655: Call to non-readonly member 'ToString' from a 'readonly' member results in an implicit copy of 'this'.
+                // (12,9): warning CS8655: Call to non-readonly member 'S.ToString()' from a 'readonly' member results in an implicit copy of 'this'.
                 //         ToString();
-                Diagnostic(ErrorCode.WRN_ImplicitCopyInReadOnlyMember, "ToString").WithArguments("ToString", "this").WithLocation(12, 9),
-                // (13,9): warning CS8655: Call to non-readonly member 'GetHashCode' from a 'readonly' member results in an implicit copy of 'this'.
+                Diagnostic(ErrorCode.WRN_ImplicitCopyInReadOnlyMember, "ToString").WithArguments("S.ToString()", "this").WithLocation(12, 9),
+                // (13,9): warning CS8655: Call to non-readonly member 'S.GetHashCode()' from a 'readonly' member results in an implicit copy of 'this'.
                 //         GetHashCode();
-                Diagnostic(ErrorCode.WRN_ImplicitCopyInReadOnlyMember, "GetHashCode").WithArguments("GetHashCode", "this").WithLocation(13, 9),
-                // (14,9): warning CS8655: Call to non-readonly member 'Equals' from a 'readonly' member results in an implicit copy of 'this'.
+                Diagnostic(ErrorCode.WRN_ImplicitCopyInReadOnlyMember, "GetHashCode").WithArguments("S.GetHashCode()", "this").WithLocation(13, 9),
+                // (14,9): warning CS8655: Call to non-readonly member 'S.Equals(object)' from a 'readonly' member results in an implicit copy of 'this'.
                 //         Equals(null);
-                Diagnostic(ErrorCode.WRN_ImplicitCopyInReadOnlyMember, "Equals").WithArguments("Equals", "this").WithLocation(14, 9));
+                Diagnostic(ErrorCode.WRN_ImplicitCopyInReadOnlyMember, "Equals").WithArguments("S.Equals(object)", "this").WithLocation(14, 9));
 
             // Verify that calls to non-readonly overrides pass the address of a temp, not the address of 'this'
             verifier.VerifyIL("S.M", @"
@@ -2348,18 +2361,18 @@ public struct S
             var verifier = CompileAndVerify(csharp);
 
             verifier.VerifyDiagnostics(
-                // (12,9): warning CS8655: Call to non-readonly member 'GetType' from a 'readonly' member results in an implicit copy of 'this'.
+                // (12,9): warning CS8655: Call to non-readonly member 'S.GetType()' from a 'readonly' member results in an implicit copy of 'this'.
                 //         GetType();
-                Diagnostic(ErrorCode.WRN_ImplicitCopyInReadOnlyMember, "GetType").WithArguments("GetType", "this").WithLocation(12, 9),
-                // (13,9): warning CS8655: Call to non-readonly member 'ToString' from a 'readonly' member results in an implicit copy of 'this'.
+                Diagnostic(ErrorCode.WRN_ImplicitCopyInReadOnlyMember, "GetType").WithArguments("S.GetType()", "this").WithLocation(12, 9),
+                // (13,9): warning CS8655: Call to non-readonly member 'S.ToString()' from a 'readonly' member results in an implicit copy of 'this'.
                 //         ToString();
-                Diagnostic(ErrorCode.WRN_ImplicitCopyInReadOnlyMember, "ToString").WithArguments("ToString", "this").WithLocation(13, 9),
-                // (14,9): warning CS8655: Call to non-readonly member 'GetHashCode' from a 'readonly' member results in an implicit copy of 'this'.
+                Diagnostic(ErrorCode.WRN_ImplicitCopyInReadOnlyMember, "ToString").WithArguments("S.ToString()", "this").WithLocation(13, 9),
+                // (14,9): warning CS8655: Call to non-readonly member 'S.GetHashCode()' from a 'readonly' member results in an implicit copy of 'this'.
                 //         GetHashCode();
-                Diagnostic(ErrorCode.WRN_ImplicitCopyInReadOnlyMember, "GetHashCode").WithArguments("GetHashCode", "this").WithLocation(14, 9),
-                // (15,9): warning CS8655: Call to non-readonly member 'Equals' from a 'readonly' member results in an implicit copy of 'this'.
+                Diagnostic(ErrorCode.WRN_ImplicitCopyInReadOnlyMember, "GetHashCode").WithArguments("S.GetHashCode()", "this").WithLocation(14, 9),
+                // (15,9): warning CS8655: Call to non-readonly member 'S.Equals(object)' from a 'readonly' member results in an implicit copy of 'this'.
                 //         Equals(null);
-                Diagnostic(ErrorCode.WRN_ImplicitCopyInReadOnlyMember, "Equals").WithArguments("Equals", "this").WithLocation(15, 9));
+                Diagnostic(ErrorCode.WRN_ImplicitCopyInReadOnlyMember, "Equals").WithArguments("S.Equals(object)", "this").WithLocation(15, 9));
 
             // Verify that calls to new non-readonly members pass an address to a temp and that calls to base members use a box.
             verifier.VerifyIL("S.M", @"

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

@@ -508,9 +508,9 @@ void M1()
 ";
             var comp = CreateCompilation(csharp, options: TestOptions.UnsafeReleaseDll);
             comp.VerifyDiagnostics(
-                // (12,25): warning CS8655: Call to non-readonly member 'GetPinnableReference' from a 'readonly' member results in an implicit copy of 'this'.
+                // (12,25): warning CS8655: Call to non-readonly member 'S1.GetPinnableReference()' from a 'readonly' member results in an implicit copy of 'this'.
                 //         fixed (int *i = this) {} // warn
-                Diagnostic(ErrorCode.WRN_ImplicitCopyInReadOnlyMember, "this").WithArguments("GetPinnableReference", "this").WithLocation(12, 25));
+                Diagnostic(ErrorCode.WRN_ImplicitCopyInReadOnlyMember, "this").WithArguments("S1.GetPinnableReference()", "this").WithLocation(12, 25));
         }
 
         [Fact]
@@ -582,9 +582,9 @@ public struct S
 {
     public int i;
 
-    public int P
+    public readonly int P
     {
-        readonly get
+        get
         {
             // should create local copy
             M();
@@ -614,9 +614,9 @@ static void Main()
 
             var verifier = CompileAndVerify(csharp, expectedOutput: "123");
             verifier.VerifyDiagnostics(
-                // (11,13): warning CS8655: Call to non-readonly member 'M' from a 'readonly' member results in an implicit copy of 'this'.
+                // (11,13): warning CS8655: Call to non-readonly member 'S.M()' from a 'readonly' member results in an implicit copy of 'this'.
                 //             M();
-                Diagnostic(ErrorCode.WRN_ImplicitCopyInReadOnlyMember, "M").WithArguments("M", "this").WithLocation(11, 13));
+                Diagnostic(ErrorCode.WRN_ImplicitCopyInReadOnlyMember, "M").WithArguments("S.M()", "this").WithLocation(11, 13));
         }
 
         [Fact]
@@ -627,9 +627,9 @@ public struct S
 {
     public int i;
 
-    public int P1
+    public readonly int P1
     {
-        readonly get
+        get
         {
             // should create local copy
             _ = P2; // warning
@@ -656,9 +656,9 @@ static void Main()
 
             var verifier = CompileAndVerify(csharp, expectedOutput: "123");
             verifier.VerifyDiagnostics(
-                // (11,17): warning CS8655: Call to non-readonly member 'get_P2' from a 'readonly' member results in an implicit copy of 'this'.
-                //             _ = P2;
-                Diagnostic(ErrorCode.WRN_ImplicitCopyInReadOnlyMember, "P2").WithArguments("get_P2", "this").WithLocation(11, 17));
+                // (11,17): warning CS8655: Call to non-readonly member 'S.P2.get' from a 'readonly' member results in an implicit copy of 'this'.
+                //             _ = P2; // warning
+                Diagnostic(ErrorCode.WRN_ImplicitCopyInReadOnlyMember, "P2").WithArguments("S.P2.get", "this").WithLocation(11, 17));
         }
 
         [Fact]
@@ -1123,7 +1123,7 @@ public struct S
 ";
             var comp = CreateCompilation(csharp);
             comp.VerifyDiagnostics(
-                // (5,32): error CS8656: Static member 'S.M()' cannot be marked 'readonly' because readonly members cannot modify 'this' and static members do not have a 'this' parameter.
+                // (5,32): error CS8656: Static member 'S.M()' cannot be marked 'readonly'.
                 //     public static readonly int M()
                 Diagnostic(ErrorCode.ERR_StaticMemberCantBeReadOnly, "M").WithArguments("S.M()").WithLocation(5, 32));
 
@@ -1139,9 +1139,9 @@ public void ReadOnlyStructProperty()
 public struct S
 {
     public int i;
-    public int P
+    public readonly int P
     {
-        readonly get
+        get
         {
             return i;
         }
@@ -1165,12 +1165,16 @@ readonly get
         {
             return i;
         }
+        set
+        {
+            i = value;
+        }
     }
 }
 ";
             var comp = CreateCompilation(csharp);
             comp.VerifyDiagnostics(
-                // (7,18): error CS8656: Static member 'S.P.get' cannot be marked 'readonly' because readonly members cannot modify 'this' and static members do not have a 'this' parameter.
+                // (7,18): error CS8656: Static member 'S.P.get' cannot be marked 'readonly'.
                 //         readonly get
                 Diagnostic(ErrorCode.ERR_StaticMemberCantBeReadOnly, "get").WithArguments("S.P.get").WithLocation(7, 18));
         }
@@ -1187,7 +1191,7 @@ public struct S
 ";
             var comp = CreateCompilation(csharp);
             comp.VerifyDiagnostics(
-                // (5,32): error CS8656: Static member 'S.P' cannot be marked 'readonly' because readonly members cannot modify 'this' and static members do not have a 'this' parameter.
+                // (5,32): error CS8656: Static member 'S.P' cannot be marked 'readonly'.
                 //     public static readonly int P => i;
                 Diagnostic(ErrorCode.ERR_StaticMemberCantBeReadOnly, "P").WithArguments("S.P").WithLocation(5, 32));
         }
@@ -1238,6 +1242,9 @@ public struct S
 ";
             var comp = CreateCompilation(csharp);
             comp.VerifyDiagnostics(
+                // (4,16): error CS8663: 'S.P1': 'readonly' can only be used on accessors if the property or indexer has both a get and a set accessor
+                //     public int P1 { readonly get; }
+                Diagnostic(ErrorCode.ERR_ReadOnlyModMissingAccessor, "P1").WithArguments("S.P1").WithLocation(4, 16),
                 // (7,16): error CS8660: Cannot specify 'readonly' modifiers on both accessors of property or indexer 'S.P4'. Instead, put a 'readonly' modifier on the property itself.
                 //     public int P4 { readonly get; readonly set; }
                 Diagnostic(ErrorCode.ERR_DuplicatePropertyReadOnlyMods, "P4").WithArguments("S.P4").WithLocation(7, 16),
@@ -1247,6 +1254,9 @@ public struct S
                 // (8,25): error CS8658: Auto-implemented property 'S.P5' cannot be marked 'readonly' because it has a 'set' accessor.
                 //     public readonly int P5 { get; set; }
                 Diagnostic(ErrorCode.ERR_AutoPropertyWithSetterCantBeReadOnly, "P5").WithArguments("S.P5").WithLocation(8, 25),
+                // (9,25): error CS8663: 'S.P6': 'readonly' can only be used on accessors if the property or indexer has both a get and a set accessor
+                //     public readonly int P6 { readonly get; }
+                Diagnostic(ErrorCode.ERR_ReadOnlyModMissingAccessor, "P6").WithArguments("S.P6").WithLocation(9, 25),
                 // (9,39): error CS8659: Cannot specify 'readonly' modifiers on both property or indexer 'S.P6' and its accessor. Remove one of them.
                 //     public readonly int P6 { readonly get; }
                 Diagnostic(ErrorCode.ERR_InvalidPropertyReadOnlyMods, "get").WithArguments("S.P6").WithLocation(9, 39),
@@ -1267,7 +1277,7 @@ public void ReadOnlyProperty_RedundantReadOnlyAccessor()
             var csharp = @"
 public struct S
 {
-    public readonly int P { readonly get => 42; }
+    public readonly int P { readonly get => 42; set {} }
 }
 ";
             var comp = CreateCompilation(csharp);
@@ -1284,15 +1294,15 @@ public void ReadOnlyStaticAutoProperty()
 public struct S
 {
     public static readonly int P1 { get; set; }
-    public static int P2 { readonly get; }
+    public static int P2 { readonly get; set; }
 }
 ";
             var comp = CreateCompilation(csharp);
             comp.VerifyDiagnostics(
-                // (4,32): error CS8656: Static member 'S.P1' cannot be marked 'readonly' because readonly members cannot modify 'this' and static members do not have a 'this' parameter.
+                // (4,32): error CS8656: Static member 'S.P1' cannot be marked 'readonly'.
                 //     public static readonly int P1 { get; set; }
                 Diagnostic(ErrorCode.ERR_StaticMemberCantBeReadOnly, "P1").WithArguments("S.P1").WithLocation(4, 32),
-                // (5,37): error CS8656: Static member 'S.P2.get' cannot be marked 'readonly' because readonly members cannot modify 'this' and static members do not have a 'this' parameter.
+                // (5,37): error CS8656: Static member 'S.P2.get' cannot be marked 'readonly'.
                 //     public static int P2 { readonly get; }
                 Diagnostic(ErrorCode.ERR_StaticMemberCantBeReadOnly, "get").WithArguments("S.P2.get").WithLocation(5, 37));
         }
@@ -1406,9 +1416,9 @@ void M1()
 ";
             var comp = CreateCompilation(csharp);
             comp.VerifyDiagnostics(
-                // (13,27): warning CS8655: Call to non-readonly member 'GetEnumerator' from a 'readonly' member results in an implicit copy of 'this'.
+                // (13,27): warning CS8655: Call to non-readonly member 'S1.GetEnumerator()' from a 'readonly' member results in an implicit copy of 'this'.
                 //         foreach (var x in this) {} // warning-- implicit copy
-                Diagnostic(ErrorCode.WRN_ImplicitCopyInReadOnlyMember, "this").WithArguments("GetEnumerator", "this").WithLocation(13, 27));
+                Diagnostic(ErrorCode.WRN_ImplicitCopyInReadOnlyMember, "this").WithArguments("S1.GetEnumerator()", "this").WithLocation(13, 27));
         }
 
         [Fact]
@@ -1469,9 +1479,9 @@ public async Task M1()
 ";
             var comp = CreateCompilationWithTasksExtensions(new[] { csharp, AsyncStreamsTypes });
             comp.VerifyDiagnostics(
-                // (16,33): warning CS8655: Call to non-readonly member 'GetAsyncEnumerator' from a 'readonly' member results in an implicit copy of 'this'.
+                // (16,33): warning CS8655: Call to non-readonly member 'S1.GetAsyncEnumerator()' from a 'readonly' member results in an implicit copy of 'this'.
                 //         await foreach (var x in this) {} // warn
-                Diagnostic(ErrorCode.WRN_ImplicitCopyInReadOnlyMember, "this").WithArguments("GetAsyncEnumerator", "this").WithLocation(16, 33));
+                Diagnostic(ErrorCode.WRN_ImplicitCopyInReadOnlyMember, "this").WithArguments("S1.GetAsyncEnumerator()", "this").WithLocation(16, 33));
         }
 
         [Fact]
@@ -1513,9 +1523,9 @@ void M1()
 ";
             var comp = CreateCompilation(csharp);
             comp.VerifyDiagnostics(
-                // (13,16): warning CS8655: Call to non-readonly member 'Dispose' from a 'readonly' member results in an implicit copy of 'this'.
+                // (13,16): warning CS8655: Call to non-readonly member 'S1.Dispose()' from a 'readonly' member results in an implicit copy of 'this'.
                 //         using (this) { } // should warn
-                Diagnostic(ErrorCode.WRN_ImplicitCopyInReadOnlyMember, "this").WithArguments("Dispose", "this").WithLocation(13, 16));
+                Diagnostic(ErrorCode.WRN_ImplicitCopyInReadOnlyMember, "this").WithArguments("S1.Dispose()", "this").WithLocation(13, 16));
         }
 
         [Fact]
@@ -1562,9 +1572,9 @@ void M1()
 ";
             var comp = CreateCompilation(csharp);
             comp.VerifyDiagnostics(
-                // (11,22): warning CS8655: Call to non-readonly member 'Deconstruct' from a 'readonly' member results in an implicit copy of 'this'.
+                // (11,22): warning CS8655: Call to non-readonly member 'S1.Deconstruct(out int, out int)' from a 'readonly' member results in an implicit copy of 'this'.
                 //         var (x, y) = this; // should warn
-                Diagnostic(ErrorCode.WRN_ImplicitCopyInReadOnlyMember, "this").WithArguments("Deconstruct", "this").WithLocation(11, 22));
+                Diagnostic(ErrorCode.WRN_ImplicitCopyInReadOnlyMember, "this").WithArguments("S1.Deconstruct(out int, out int)", "this").WithLocation(11, 22));
         }
 
         [Fact]
@@ -1758,13 +1768,23 @@ public struct S3
 public struct S4
 {
     // error
-    public readonly int this[int i]
+    public int this[int i]
     {
         readonly get { return i; }
     }
 }
 
 public struct S5
+{
+    // error
+    public readonly int this[int i]
+    {
+        readonly get { return i; }
+        set { }
+    }
+}
+
+public struct S6
 {
     // error
     public static readonly int this[int i] => i;
@@ -1775,12 +1795,15 @@ public struct S5
                 // (21,16): error CS8660: Cannot specify 'readonly' modifiers on both accessors of property or indexer 'S3.this[int]'. Instead, put a 'readonly' modifier on the property itself.
                 //     public int this[int i]
                 Diagnostic(ErrorCode.ERR_DuplicatePropertyReadOnlyMods, "this").WithArguments("S3.this[int]").WithLocation(21, 16),
-                // (33,18): error CS8659: Cannot specify 'readonly' modifiers on both property or indexer 'S4.this[int]' and its accessor. Remove one of them.
+                // (31,16): error CS8663: 'S4.this[int]': 'readonly' can only be used on accessors if the property or indexer has both a get and a set accessor
+                //     public int this[int i]
+                Diagnostic(ErrorCode.ERR_ReadOnlyModMissingAccessor, "this").WithArguments("S4.this[int]").WithLocation(31, 16),
+                // (42,18): error CS8659: Cannot specify 'readonly' modifiers on both property or indexer 'S5.this[int]' and its accessor. Remove one of them.
                 //         readonly get { return i; }
-                Diagnostic(ErrorCode.ERR_InvalidPropertyReadOnlyMods, "get").WithArguments("S4.this[int]").WithLocation(33, 18),
-                // (40,32): error CS0106: The modifier 'static' is not valid for this item
+                Diagnostic(ErrorCode.ERR_InvalidPropertyReadOnlyMods, "get").WithArguments("S5.this[int]").WithLocation(42, 18),
+                // (50,32): error CS0106: The modifier 'static' is not valid for this item
                 //     public static readonly int this[int i] => i;
-                Diagnostic(ErrorCode.ERR_BadMemberFlag, "this").WithArguments("static").WithLocation(40, 32));
+                Diagnostic(ErrorCode.ERR_BadMemberFlag, "this").WithArguments("static").WithLocation(50, 32));
         }
 
         [Fact]
@@ -1838,7 +1861,7 @@ public struct S1
 ";
             var comp = CreateCompilation(csharp);
             comp.VerifyDiagnostics(
-                // (6,52): error CS8656: Static member 'S1.E' cannot be marked 'readonly' because readonly members cannot modify 'this' and static members do not have a 'this' parameter.
+                // (6,52): error CS8656: Static member 'S1.E' cannot be marked 'readonly'.
                 //     public static readonly event Action<EventArgs> E
                 Diagnostic(ErrorCode.ERR_StaticMemberCantBeReadOnly, "E").WithArguments("S1.E").WithLocation(6, 52));
         }
@@ -1879,11 +1902,11 @@ public struct S
     public readonly void M() {}
 
     public readonly int P1 => 42;
-    public int P2 { readonly get => 123; }
-    public int P3 { readonly set {} }
+    public int P2 { readonly get => 123; set {} }
+    public int P3 { get => 123; readonly set {} }
 
     public readonly int this[int i] => i;
-    public int this[int i, int j] { readonly get => i + j; }
+    public int this[int i, int j] { readonly get => i + j; set {} }
 
     public readonly event Action<EventArgs> E { add {} remove {} }
 }
@@ -1891,22 +1914,22 @@ public struct S
             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() {}
+                //     public readonly void M() {}
                 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; }
+                //     public int P2 { readonly get => 123; set {} }
                 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),
+                // (10,33): error CS8652: The feature 'readonly members' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.
+                //     public int P3 { get => 123; readonly set {} }
+                Diagnostic(ErrorCode.ERR_FeatureInPreview, "readonly").WithArguments("readonly members").WithLocation(10, 33),
                 // (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; }
+                //     public int this[int i, int j] { readonly get => i + j; set {} }
                 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 {} }
@@ -1915,5 +1938,46 @@ public struct S
             comp = CreateCompilation(csharp);
             comp.VerifyDiagnostics();
         }
+
+        [Fact]
+        public void ReadOnlyMethod_CompoundPropertyAssignment()
+        {
+            var csharp = @"
+struct S
+{
+    int P1 { get => 123; set {} }
+    int P2 { readonly get => 123; set {} }
+    int P3 { get => 123; readonly set {} }
+    readonly int P4 { get => 123; set {} }
+
+    void M1()
+    {
+        // ok
+        P1 += 1;
+        P2 += 1;
+        P3 += 1;
+        P4 += 1;
+    }
+
+    readonly void M2()
+    {
+        P1 += 1; // error
+        P2 += 1; // error
+        P3 += 1; // warning
+        P4 += 1; // ok
+    }
+}";
+            var comp = CreateCompilation(csharp);
+            comp.VerifyDiagnostics(
+                // (20,9): error CS1604: Cannot assign to 'P1' because it is read-only
+                //         P1 += 1; // error
+                Diagnostic(ErrorCode.ERR_AssgReadonlyLocal, "P1").WithArguments("P1").WithLocation(20, 9),
+                // (21,9): error CS1604: Cannot assign to 'P2' because it is read-only
+                //         P2 += 1; // error
+                Diagnostic(ErrorCode.ERR_AssgReadonlyLocal, "P2").WithArguments("P2").WithLocation(21, 9),
+                // (22,9): warning CS8655: Call to non-readonly member 'S.P3.get' from a 'readonly' member results in an implicit copy of 'this'.
+                //         P3 += 1; // warning
+                Diagnostic(ErrorCode.WRN_ImplicitCopyInReadOnlyMember, "P3").WithArguments("S.P3.get", "this").WithLocation(22, 9));
+        }
     }
 }

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

@@ -2907,6 +2907,27 @@ public void TestStructGetterPropertyWithReadonly()
             Assert.Equal(SyntaxKind.ReadOnlyKeyword, accessors[0].Modifiers[0].Kind());
         }
 
+        [Fact]
+        public void TestStructBadExpressionProperty()
+        {
+            var text =
+@"public struct S
+{
+    public int P readonly => 0;
+}
+";
+            var file = this.ParseFile(text, TestOptions.Regular);
+
+            Assert.NotNull(file);
+            Assert.Equal(1, file.Members.Count);
+            Assert.Equal(text, file.ToString());
+
+            Assert.Equal(3, file.Errors().Length);
+            Assert.Equal(ErrorCode.ERR_SemicolonExpected, (ErrorCode)file.Errors()[0].Code);
+            Assert.Equal(ErrorCode.ERR_InvalidMemberDecl, (ErrorCode)file.Errors()[1].Code);
+            Assert.Equal(ErrorCode.ERR_InvalidMemberDecl, (ErrorCode)file.Errors()[2].Code);
+        }
+
         [Fact]
         public void TestClassMethodWithParameter()
         {