Fix auto-default behavior when value-assigning a ref field

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -7784,4 +7784,10 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="WRN_TargetDifferentRefness_Title" xml:space="preserve">
     <value>Reference kind modifier of parameter doesn't match the corresponding parameter in target.</value>
   </data>
+  <data name="WRN_UseDefViolationRefField" xml:space="preserve">
+    <value>Ref field '{0}' should be ref-assigned before use.</value>
+  </data>
+  <data name="WRN_UseDefViolationRefField_Title" xml:space="preserve">
+    <value>Ref field should be ref-assigned before use.</value>
+  </data>
 </root>

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

@@ -2265,6 +2265,7 @@ internal enum ErrorCode
         WRN_TargetDifferentRefness = 9198,
         ERR_OutAttrOnRefReadonlyParam = 9199,
         WRN_RefReadonlyParameterDefaultValue = 9200,
+        WRN_UseDefViolationRefField = 9201,
 
         #endregion
 

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

@@ -550,6 +550,7 @@ internal static int GetWarningLevel(ErrorCode code)
                 case ErrorCode.WRN_HidingDifferentRefness:
                 case ErrorCode.WRN_TargetDifferentRefness:
                 case ErrorCode.WRN_RefReadonlyParameterDefaultValue:
+                case ErrorCode.WRN_UseDefViolationRefField:
                     return 1;
                 default:
                     return 0;
@@ -2396,6 +2397,7 @@ internal static bool IsBuildOnlyDiagnostic(ErrorCode code)
                 case ErrorCode.ERR_OutAttrOnRefReadonlyParam:
                 case ErrorCode.WRN_RefReadonlyParameterDefaultValue:
                 case ErrorCode.WRN_ByValArraySizeConstRequired:
+                case ErrorCode.WRN_UseDefViolationRefField:
                     return false;
                 default:
                     // NOTE: All error codes must be explicitly handled in this switch statement

src/Compilers/CSharp/Portable/FlowAnalysis/DefiniteAssignment.cs

@@ -1324,7 +1324,18 @@ void addDiagnosticForStructField(int fieldSlot, FieldSymbol fieldSymbol)
                         // should we handle nested fields here? https://github.com/dotnet/roslyn/issues/59890
                         AddImplicitlyInitializedField((FieldSymbol)fieldIdentifier.Symbol);
 
-                        if (compilation.IsFeatureEnabled(MessageID.IDS_FeatureAutoDefaultStructs))
+                        if (fieldSymbol.RefKind != RefKind.None)
+                        {
+                            // 'hasAssociatedProperty' is only true here in error scenarios where we don't need to report this as a cascading diagnostic
+                            if (!hasAssociatedProperty)
+                            {
+                                Diagnostics.Add(
+                                    ErrorCode.WRN_UseDefViolationRefField,
+                                    node.Location,
+                                    symbolName);
+                            }
+                        }
+                        else if (compilation.IsFeatureEnabled(MessageID.IDS_FeatureAutoDefaultStructs))
                         {
                             Diagnostics.Add(
                                 hasAssociatedProperty ? ErrorCode.WRN_UseDefViolationPropertySupportedVersion : ErrorCode.WRN_UseDefViolationFieldSupportedVersion,
@@ -1497,6 +1508,10 @@ private Symbol UseNonFieldSymbolUnsafely(BoundExpression expression)
 
         protected void Assign(BoundNode node, BoundExpression value, bool isRef = false, bool read = true)
         {
+            if (!isRef && node is BoundFieldAccess { FieldSymbol.RefKind: not RefKind.None } fieldAccess)
+            {
+                CheckAssigned(fieldAccess, node.Syntax);
+            }
             AssignImpl(node, value, written: true, isRef: isRef, read: read);
         }