special handling for initializers

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -8003,7 +8003,7 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
     <value>Non-nullable property must contain a non-null value when exiting constructor. Consider adding the 'required' modifier, or declaring the property as nullable, or adding '[field: MaybeNull, AllowNull]' attributes.</value>
   </data>
   <data name="WRN_AccessorDoesNotUseBackingField" xml:space="preserve">
-    <value>The '{0}' accessor of property '{1}' should use the backing 'field' because the other accessor is using it.</value>
+    <value>'{0}' should use the backing 'field' because the other accessor is using it.</value>
   </data>
   <data name="WRN_AccessorDoesNotUseBackingField_Title" xml:space="preserve">
     <value>Property accessor should use the backing 'field' because the other accessor is using it.</value>

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

@@ -74,7 +74,7 @@ private static SourcePropertySymbol Create(
             string? aliasQualifierOpt;
             string memberName = ExplicitInterfaceHelpers.GetMemberNameAndInterfaceSymbol(binder, explicitInterfaceSpecifier, name, diagnostics, out explicitInterfaceType, out aliasQualifierOpt);
 
-            var symbol = new SourcePropertySymbol(
+            return new SourcePropertySymbol(
                 containingType,
                 syntax,
                 hasGetAccessor: getSyntax != null || isExpressionBodied,
@@ -88,30 +88,11 @@ private static SourcePropertySymbol Create(
                 hasAutoPropertySet: hasAutoPropertySet,
                 isExpressionBodied: isExpressionBodied,
                 accessorsHaveImplementation: accessorsHaveImplementation,
-                usesFieldKeyword: getterUsesFieldKeyword || setterUsesFieldKeyword,
+                getterUsesFieldKeyword: getterUsesFieldKeyword,
+                setterUsesFieldKeyword: setterUsesFieldKeyword,
                 memberName,
                 location,
                 diagnostics);
-
-            if (binder.Compilation.IsFeatureEnabled(MessageID.IDS_FeatureFieldKeyword))
-            {
-                AccessorDeclarationSyntax? accessorToBlame = null;
-                if (hasSetAccessorImplementation && !setterUsesFieldKeyword && (hasAutoPropertyGet || getterUsesFieldKeyword))
-                {
-                    accessorToBlame = setSyntax;
-                }
-                else if (hasGetAccessorImplementation && !getterUsesFieldKeyword && (hasAutoPropertySet || setterUsesFieldKeyword))
-                {
-                    accessorToBlame = getSyntax;
-                }
-
-                if (accessorToBlame is not null)
-                {
-                    diagnostics.Add(ErrorCode.WRN_AccessorDoesNotUseBackingField, accessorToBlame.Keyword, [accessorToBlame.Keyword.ValueText, symbol]);
-                }
-            }
-
-            return symbol;
         }
 
         private SourcePropertySymbol(
@@ -128,7 +109,8 @@ private SourcePropertySymbol(
             bool hasAutoPropertySet,
             bool isExpressionBodied,
             bool accessorsHaveImplementation,
-            bool usesFieldKeyword,
+            bool getterUsesFieldKeyword,
+            bool setterUsesFieldKeyword,
             string memberName,
             Location location,
             BindingDiagnosticBag diagnostics)
@@ -147,7 +129,8 @@ private SourcePropertySymbol(
                 hasAutoPropertySet: hasAutoPropertySet,
                 isExpressionBodied: isExpressionBodied,
                 accessorsHaveImplementation: accessorsHaveImplementation,
-                usesFieldKeyword: usesFieldKeyword,
+                getterUsesFieldKeyword: getterUsesFieldKeyword,
+                setterUsesFieldKeyword: setterUsesFieldKeyword,
                 syntax.Type.SkipScoped(out _).GetRefKindInLocalOrReturn(diagnostics),
                 memberName,
                 syntax.AttributeLists,

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

@@ -31,12 +31,13 @@ private enum Flags : ushort
             IsExpressionBodied = 1 << 0,
             HasAutoPropertyGet = 1 << 1,
             HasAutoPropertySet = 1 << 2,
-            UsesFieldKeyword = 1 << 3,
-            IsExplicitInterfaceImplementation = 1 << 4,
-            HasInitializer = 1 << 5,
-            AccessorsHaveImplementation = 1 << 6,
-            HasExplicitAccessModifier = 1 << 7,
-            RequiresBackingField = 1 << 8,
+            GetterUsesFieldKeyword = 1 << 4,
+            SetterUsesFieldKeyword = 1 << 5,
+            IsExplicitInterfaceImplementation = 1 << 6,
+            HasInitializer = 1 << 7,
+            AccessorsHaveImplementation = 1 << 8,
+            HasExplicitAccessModifier = 1 << 9,
+            RequiresBackingField = 1 << 10,
         }
 
         // TODO (tomat): consider splitting into multiple subclasses/rare data.
@@ -90,7 +91,8 @@ protected SourcePropertySymbolBase(
             bool hasAutoPropertySet,
             bool isExpressionBodied,
             bool accessorsHaveImplementation,
-            bool usesFieldKeyword,
+            bool getterUsesFieldKeyword,
+            bool setterUsesFieldKeyword,
             RefKind refKind,
             string memberName,
             SyntaxList<AttributeListSyntax> indexerNameAttributeLists,
@@ -133,9 +135,14 @@ protected SourcePropertySymbolBase(
                 _propertyFlags |= Flags.HasAutoPropertySet;
             }
 
-            if (usesFieldKeyword)
+            if (getterUsesFieldKeyword)
             {
-                _propertyFlags |= Flags.UsesFieldKeyword;
+                _propertyFlags |= Flags.GetterUsesFieldKeyword;
+            }
+
+            if (setterUsesFieldKeyword)
+            {
+                _propertyFlags |= Flags.SetterUsesFieldKeyword;
             }
 
             if (hasInitializer)
@@ -171,7 +178,7 @@ protected SourcePropertySymbolBase(
                 _name = _lazySourceName = memberName;
             }
 
-            if (usesFieldKeyword || hasAutoPropertyGet || hasAutoPropertySet || hasInitializer)
+            if (getterUsesFieldKeyword || setterUsesFieldKeyword || hasAutoPropertyGet || hasAutoPropertySet || hasInitializer)
             {
                 Debug.Assert(!IsIndexer);
                 _propertyFlags |= Flags.RequiresBackingField;
@@ -305,6 +312,39 @@ protected void CheckInitializerIfNeeded(BindingDiagnosticBag diagnostics)
             }
         }
 
+#nullable enable
+        private void CheckFieldKeywordUsage(BindingDiagnosticBag diagnostics)
+        {
+            if (!this.DeclaringCompilation.IsFeatureEnabled(MessageID.IDS_FeatureFieldKeyword))
+            {
+                return;
+            }
+
+            check((SourcePropertySymbolBase?)PartialImplementationPart ?? this, diagnostics);
+
+            static void check(SourcePropertySymbolBase @this, BindingDiagnosticBag diagnostics)
+            {
+                SourcePropertyAccessorSymbol? accessorToBlame = null;
+                var propertyFlags = @this._propertyFlags;
+                var getterUsesFieldKeyword = (propertyFlags & Flags.GetterUsesFieldKeyword) != 0;
+                var setterUsesFieldKeyword = (propertyFlags & Flags.SetterUsesFieldKeyword) != 0;
+                if (@this._setMethod is { IsAutoPropertyAccessor: false } setMethod && !setterUsesFieldKeyword && !@this.IsSetOnEitherPart(Flags.HasInitializer) && (@this.HasAutoPropertyGet || getterUsesFieldKeyword))
+                {
+                    accessorToBlame = setMethod;
+                }
+                else if (@this._getMethod is { IsAutoPropertyAccessor: false } getMethod && !getterUsesFieldKeyword && (@this.HasAutoPropertySet || setterUsesFieldKeyword))
+                {
+                    accessorToBlame = getMethod;
+                }
+
+                if (accessorToBlame is not null)
+                {
+                    diagnostics.Add(ErrorCode.WRN_AccessorDoesNotUseBackingField, accessorToBlame.GetFirstLocation(), accessorToBlame);
+                }
+            }
+        }
+#nullable disable
+
         public sealed override RefKind RefKind
         {
             get
@@ -648,10 +688,10 @@ public bool HasSkipLocalsInitAttribute
         }
 
         internal bool IsAutoPropertyOrUsesFieldKeyword
-            => IsSetOnEitherPart(Flags.HasAutoPropertyGet | Flags.HasAutoPropertySet | Flags.UsesFieldKeyword);
+            => IsSetOnEitherPart(Flags.HasAutoPropertyGet | Flags.HasAutoPropertySet | Flags.GetterUsesFieldKeyword | Flags.SetterUsesFieldKeyword);
 
         internal bool UsesFieldKeyword
-            => IsSetOnEitherPart(Flags.UsesFieldKeyword);
+            => IsSetOnEitherPart(Flags.GetterUsesFieldKeyword | Flags.SetterUsesFieldKeyword);
 
         protected bool HasExplicitAccessModifier
             => (_propertyFlags & Flags.HasExplicitAccessModifier) != 0;
@@ -811,6 +851,7 @@ internal override void AfterAddingTypeMembersChecks(ConversionsBase conversions,
             this.CheckModifiers(isExplicitInterfaceImplementation, Location, IsIndexer, diagnostics);
 
             CheckInitializerIfNeeded(diagnostics);
+            CheckFieldKeywordUsage(diagnostics);
 
             if (RefKind != RefKind.None && IsRequired)
             {

src/Compilers/CSharp/Portable/Symbols/Synthesized/Records/SynthesizedRecordEqualityContractProperty.cs

@@ -36,7 +36,8 @@ public SynthesizedRecordEqualityContractProperty(SourceMemberContainerTypeSymbol
                 hasAutoPropertySet: false,
                 isExpressionBodied: false,
                 accessorsHaveImplementation: true,
-                usesFieldKeyword: false,
+                getterUsesFieldKeyword: false,
+                setterUsesFieldKeyword: false,
                 RefKind.None,
                 PropertyName,
                 indexerNameAttributeLists: new SyntaxList<AttributeListSyntax>(),

src/Compilers/CSharp/Portable/Symbols/Synthesized/Records/SynthesizedRecordPropertySymbol.cs

@@ -34,7 +34,8 @@ public SynthesizedRecordPropertySymbol(
                 hasAutoPropertySet: true,
                 isExpressionBodied: false,
                 accessorsHaveImplementation: true,
-                usesFieldKeyword: false,
+                getterUsesFieldKeyword: false,
+                setterUsesFieldKeyword: false,
                 RefKind.None,
                 backingParameter.Name,
                 indexerNameAttributeLists: new SyntaxList<AttributeListSyntax>(),