Restore removed extensible enum members via back compat

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/LibraryVisitor.cs

@@ -297,7 +297,7 @@ protected internal virtual FinallyExpression VisitFinallyExpression(FinallyExpre
         /// </summary>
         /// <param name="property">The original <see cref="PropertyProvider"/>.</param>
         /// <returns>Null if it should be removed otherwise the modified version of the <see cref="PropertyProvider"/>.</returns>
-        protected virtual PropertyProvider? VisitProperty(PropertyProvider property)
+        protected internal virtual PropertyProvider? VisitProperty(PropertyProvider property)
         {
             return property;
         }

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Providers/EnumProvider.cs

@@ -68,7 +68,7 @@ protected override string BuildNamespace() => string.IsNullOrEmpty(_inputType?.N
         protected static string RemoveUnderscores(string name) => name.Replace("_", string.Empty);
 
         private HashSet<string>? _customMemberNames;
-        private HashSet<string> CustomMemberNames => _customMemberNames ??= new HashSet<string>(
+        private protected HashSet<string> CustomMemberNames => _customMemberNames ??= new HashSet<string>(
             GetCustomMemberNames(),
             StringComparer.OrdinalIgnoreCase);
 

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Providers/ExtensibleEnumProvider.cs

@@ -4,8 +4,10 @@
 using System;
 using System.Collections.Generic;
 using System.ComponentModel;
+using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
 using System.Linq;
+using Microsoft.TypeSpec.Generator.EmitterRpc;
 using Microsoft.TypeSpec.Generator.Expressions;
 using Microsoft.TypeSpec.Generator.Input;
 using Microsoft.TypeSpec.Generator.Input.Extensions;
@@ -274,5 +276,82 @@ protected override TypeProvider[] BuildSerializationProviders()
             return CodeModelGenerator.Instance.TypeFactory.CreateSerializations(_inputType, this).ToArray();
         }
         protected override bool GetIsEnum() => true;
+
+        protected internal override IReadOnlyList<EnumTypeMember>? BuildEnumValuesForBackCompatibility(IReadOnlyList<EnumTypeMember> currentValues)
+        {
+            var lastContractProperties = LastContractView?.Properties;
+            if (lastContractProperties == null || lastContractProperties.Count == 0)
+            {
+                return null;
+            }
+
+            var currentNames = new HashSet<string>(currentValues.Select(v => v.Name), StringComparer.OrdinalIgnoreCase);
+            var lastContractValueFields = new Dictionary<string, FieldProvider>(StringComparer.Ordinal);
+            foreach (var field in LastContractView!.Fields)
+            {
+                lastContractValueFields.TryAdd(field.Name, field);
+            }
+
+            List<EnumTypeMember>? restoredMembers = null;
+            foreach (var property in lastContractProperties)
+            {
+                // Members that still exist in the current spec or are provided by custom code are left untouched.
+                if (currentNames.Contains(property.Name) || CustomMemberNames.Contains(property.Name))
+                {
+                    continue;
+                }
+
+                // Honor an intentional removal recorded in the ApiCompat baseline.
+                if (CodeModelGenerator.Instance.SourceInputModel?.ApiCompatBaseline.IsMemberSuppressed(Type.FullyQualifiedName, property.Name, 0) == true)
+                {
+                    CodeModelGenerator.Instance.Emitter.Debug(
+                        $"Skipping re-add of enum member '{Name}.{property.Name}'; the removal is accepted in the ApiCompat baseline.",
+                        BackCompatibilityChangeCategory.BaselineAcceptedRemovalSkipped);
+                    continue;
+                }
+
+                if (TryResurrectRemovedMember(property, lastContractValueFields, out var resurrectedMember))
+                {
+                    (restoredMembers ??= []).Add(resurrectedMember);
+                    CodeModelGenerator.Instance.Emitter.Debug(
+                        $"Re-added enum member '{property.Name}' to enum '{Name}' to preserve a member from the last contract.",
+                        BackCompatibilityChangeCategory.EnumMemberAddedFromLastContract);
+                }
+            }
+
+            if (restoredMembers == null)
+            {
+                return null;
+            }
+
+            // Preserve the current spec order and append the restored members at the end.
+            return [.. currentValues, .. restoredMembers];
+        }
+
+        private bool TryResurrectRemovedMember(
+            PropertyProvider lastContractProperty,
+            IReadOnlyDictionary<string, FieldProvider> lastContractValueFields,
+            [NotNullWhen(true)] out EnumTypeMember? member)
+        {
+            member = null;
+
+            // The wire value lives in the private const `<Member>Value` field.
+            var valueFieldName = $"{lastContractProperty.Name}Value";
+            if (!lastContractValueFields.TryGetValue(valueFieldName, out var valueField)
+                || valueField.InitializationValue is not LiteralExpression { Literal: { } literalValue })
+            {
+                return false;
+            }
+
+            var field = new FieldProvider(
+                FieldModifiers.Private | FieldModifiers.Const,
+                EnumUnderlyingType,
+                valueFieldName,
+                this,
+                lastContractProperty.Description,
+                Literal(literalValue));
+            member = new EnumTypeMember(lastContractProperty.Name, field, literalValue);
+            return true;
+        }
     }
 }

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Providers/NamedTypeSymbolProvider.cs

@@ -249,13 +249,9 @@ protected internal override PropertyProvider[] BuildProperties()
 
         private static ValueExpression? GetFieldInitializer(IFieldSymbol fieldSymbol)
         {
-            if (fieldSymbol.ContainingType?.TypeKind == TypeKind.Enum)
+            if (fieldSymbol.HasConstantValue && fieldSymbol.ConstantValue != null)
             {
-                if (fieldSymbol.HasConstantValue && fieldSymbol.ConstantValue != null)
-                {
-                    return Literal(fieldSymbol.ConstantValue);
-                }
-                return null;
+                return Literal(fieldSymbol.ConstantValue);
             }
 
             return null;

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Providers/TypeProvider.cs

@@ -825,7 +825,8 @@ internal void ProcessTypeForBackCompatibility()
 
             IReadOnlyList<EnumTypeMember>? updatedEnumValues = null;
             IEnumerable<FieldProvider>? newFields = null;
-            if (this is EnumProvider)
+            IEnumerable<PropertyProvider>? newProperties = null;
+            if (this is EnumProvider enumProvider)
             {
                 var hasFields = LastContractView?.Fields != null && LastContractView.Fields.Count > 0;
                 if (hasFields)
@@ -848,21 +849,48 @@ internal void ProcessTypeForBackCompatibility()
                             updatedEnumValues = newEnumValues;
                         }
 
-                        newFields = filteredFields;
+                        // Sync the enum values before rebuilding the member collections from them.
+                        if (updatedEnumValues != null)
+                        {
+                            _enumValues = updatedEnumValues;
+                        }
+
+                        if (enumProvider.IsExtensible)
+                        {
+                            // Extensible enums carry an extra backing `_value` field and surface members
+                            // as properties, so rebuild both from the updated members. Reuse the
+                            // already-visited field and property instances for members that still exist so
+                            // any visitor mutations are preserved and only the restored members are
+                            // (re)visited below.
+                            var existingFields = new Dictionary<string, FieldProvider>(StringComparer.Ordinal);
+                            foreach (var field in Fields)
+                            {
+                                existingFields.TryAdd(field.Name, field);
+                            }
+                            newFields = ApplyCustomizationFilter(
+                                BuildFields().Select(f => existingFields.TryGetValue(f.Name, out var existing) ? existing : f));
+
+                            var existingProperties = new Dictionary<string, PropertyProvider>(StringComparer.Ordinal);
+                            foreach (var property in Properties)
+                            {
+                                existingProperties.TryAdd(property.Name, property);
+                            }
+                            newProperties = ApplyCustomizationFilter(
+                                BuildProperties().Select(p => existingProperties.TryGetValue(p.Name, out var existing) ? existing : p));
+                        }
+                        else
+                        {
+                            newFields = filteredFields;
+                        }
                     }
                 }
             }
 
             var newMethods = hasMethods ? BuildMethodsForBackCompatibility(Methods) : null;
             var newConstructors = hasConstructors ? BuildConstructorsForBackCompatibility(Constructors) : null;
 
-            if (newFields != null || newMethods != null || newConstructors != null)
+            if (newFields != null || newProperties != null || newMethods != null || newConstructors != null)
             {
-                if (updatedEnumValues != null)
-                {
-                    _enumValues = updatedEnumValues;
-                }
-
                 // Back-compatibility processing intentionally runs after the library visitor pass so
                 // that the contract comparison uses the final, post-visitor member signatures (otherwise
                 // we could incorrectly decide whether a back-compat member is needed). As a result, any
@@ -877,12 +905,16 @@ internal void ProcessTypeForBackCompatibility()
                 {
                     newConstructors = VisitNewMembers(newConstructors, Constructors, static (member, visitor) => visitor.VisitConstructor(member));
                 }
+                if (newProperties != null)
+                {
+                    newProperties = VisitNewMembers(newProperties, Properties, static (member, visitor) => visitor.VisitProperty(member));
+                }
                 if (newFields != null)
                 {
                     newFields = VisitNewMembers(newFields, Fields, static (member, visitor) => visitor.VisitField(member));
                 }
 
-                Update(fields: newFields, methods: newMethods, constructors: newConstructors);
+                Update(fields: newFields, properties: newProperties, methods: newMethods, constructors: newConstructors);
             }
 
             // Providers whose attributes depend on final generation decisions build their attributes at write

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/OutputLibraryVisitorTests.cs

@@ -494,7 +494,7 @@ private class TestFilterVisitor : LibraryVisitor
                 return constructor;
             }
 
-            protected override PropertyProvider? VisitProperty(PropertyProvider property)
+            protected internal override PropertyProvider? VisitProperty(PropertyProvider property)
             {
                 if (property.Name == "TestProperty")
                 {