Fix Back Compat for Discriminated Model Factory Methods

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

@@ -328,8 +328,8 @@ private bool TryBuildCompatibleMethodForPreviousContract(
                     continue;
                 }
 
-                var model = GetModelToInstantiateForFactoryMethod(modelProvider);
-                if (model != null && previousMethodReturnType.AreNamesEqual(model.Type))
+                var model = GetModelToInstantiateForPreviousReturnType(modelProvider, previousMethodReturnType);
+                if (model != null)
                 {
                     modelToInstantiate = model;
                     break;
@@ -525,6 +525,26 @@ private static IReadOnlyList<ValueExpression> GetCtorArgs(
                 : modelProvider;
         }
 
+        private static ModelProvider? GetModelToInstantiateForPreviousReturnType(ModelProvider modelProvider, CSharpType returnType)
+        {
+            var model = GetModelToInstantiateForFactoryMethod(modelProvider);
+            if (model is null)
+            {
+                return null;
+            }
+
+            if (modelProvider.DeclarationModifiers.HasFlag(TypeSignatureModifiers.Abstract))
+            {
+                return returnType.AreNamesEqual(modelProvider.Type) || returnType.AreNamesEqual(model.Type)
+                    ? model
+                    : null;
+            }
+
+            return returnType.AreNamesEqual(model.Type)
+                ? model
+                : null;
+        }
+
         private static (ParameterProvider? BinaryDataParam, ConstructorProvider FullCtor) GetBinaryDataParamAndFullCtorForFactoryMethod(
             ModelProvider modelProvider)
         {

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/ModelFactories/ModelFactoryProviderTests.cs

@@ -1068,6 +1068,57 @@ public async Task BackCompatibility_SwapTypeParamsDoesNotCorrupt()
                 result);
         }
 
+        [Test]
+        public async Task BackCompatibility_AbstractReturnTypeOverloadIsGenerated()
+        {
+            var derived = InputFactory.Model("DerivedModel", discriminatedKind: "derived");
+            var baseModel = InputFactory.Model(
+                "AbstractModel",
+                properties:
+                [
+                    InputFactory.Property("kind", InputPrimitiveType.String, isRequired: true, isDiscriminator: true),
+                    InputFactory.Property("prop1", InputPrimitiveType.String),
+                    InputFactory.Property("prop2", InputPrimitiveType.String),
+                ],
+                derivedModels: [derived]);
+
+            _instance = (await MockHelpers.LoadMockGeneratorAsync(
+                inputNamespaceName: "Sample.Namespace",
+                inputModelTypes: [baseModel, derived],
+                lastContractCompilation: async () => await Helpers.GetCompilationFromDirectoryAsync())).Object;
+
+            var modelFactory = _instance!.OutputLibrary.ModelFactory.Value;
+            modelFactory.ProcessTypeForBackCompatibility();
+
+            var content = new TypeProviderWriter(modelFactory).Write().Content;
+            Assert.AreEqual(Helpers.GetExpectedFromFile(), content);
+        }
+
+        [Test]
+        public async Task BackCompatibility_UnknownDiscriminatorReturnTypeOverloadIsGenerated()
+        {
+            var derived = InputFactory.Model("DerivedModel", discriminatedKind: "derived");
+            var baseModel = InputFactory.Model(
+                "AbstractModel",
+                properties:
+                [
+                    InputFactory.Property("kind", InputPrimitiveType.String, isRequired: true, isDiscriminator: true),
+                    InputFactory.Property("prop1", InputPrimitiveType.String),
+                ],
+                derivedModels: [derived]);
+
+            _instance = (await MockHelpers.LoadMockGeneratorAsync(
+                inputNamespaceName: "Sample.Namespace",
+                inputModelTypes: [baseModel, derived],
+                lastContractCompilation: async () => await Helpers.GetCompilationFromDirectoryAsync())).Object;
+
+            var modelFactory = _instance!.OutputLibrary.ModelFactory.Value;
+            modelFactory.ProcessTypeForBackCompatibility();
+
+            var content = new TypeProviderWriter(modelFactory).Write().Content;
+            Assert.AreEqual(Helpers.GetExpectedFromFile(), content);
+        }
+
         [Test]
         public void RequiredConstantPropertiesAreNotExposedAsParameters()
         {

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/ModelFactories/TestData/ModelFactoryProviderTests/BackCompatibility_AbstractReturnTypeOverloadIsGenerated.cs

@@ -0,0 +1,28 @@
+// <auto-generated/>
+
+#nullable disable
+
+using System.ComponentModel;
+using Sample.Models;
+
+namespace Sample.Namespace
+{
+    public static partial class SampleNamespaceModelFactory
+    {
+        public static global::Sample.Models.AbstractModel AbstractModel(string kind = default, string prop1 = default, string prop2 = default)
+        {
+            return new global::Sample.Models.UnknownAbstractModel(kind, prop1, prop2, additionalBinaryDataProperties: null);
+        }
+
+        public static global::Sample.Models.DerivedModel DerivedModel(string prop1 = default, string prop2 = default, string stringProperty = default)
+        {
+            return new global::Sample.Models.DerivedModel("derived", prop1, prop2, additionalBinaryDataProperties: null, stringProperty);
+        }
+
+        [global::System.ComponentModel.EditorBrowsableAttribute(global::System.ComponentModel.EditorBrowsableState.Never)]
+        public static global::Sample.Models.AbstractModel AbstractModel(string prop1, string kind)
+        {
+            return AbstractModel(kind: kind, prop1: prop1, prop2: default);
+        }
+    }
+}

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/ModelFactories/TestData/ModelFactoryProviderTests/BackCompatibility_AbstractReturnTypeOverloadIsGenerated/SampleNamespaceModelFactory.cs

@@ -0,0 +1,19 @@
+using Sample.Models;
+
+namespace Sample.Namespace
+{
+    public static partial class SampleNamespaceModelFactory
+    {
+        // Previous contract shipped a factory overload for the abstract AbstractModel with fewer
+        // parameters (before prop2 was added). It must be restored as a hidden back-compat overload
+        // even though the current factory instantiates the Unknown* derived type.
+        public static AbstractModel AbstractModel(string prop1 = default, string kind = default)
+        { }
+    }
+}
+
+namespace Sample.Models
+{
+    public abstract partial class AbstractModel
+    { }
+}

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/ModelFactories/TestData/ModelFactoryProviderTests/BackCompatibility_UnknownDiscriminatorReturnTypeOverloadIsGenerated.cs

@@ -0,0 +1,28 @@
+// <auto-generated/>
+
+#nullable disable
+
+using System.ComponentModel;
+using Sample.Models;
+
+namespace Sample.Namespace
+{
+    public static partial class SampleNamespaceModelFactory
+    {
+        public static global::Sample.Models.AbstractModel AbstractModel(string kind = default, string prop1 = default)
+        {
+            return new global::Sample.Models.UnknownAbstractModel(kind, prop1, additionalBinaryDataProperties: null);
+        }
+
+        public static global::Sample.Models.DerivedModel DerivedModel(string prop1 = default, string stringProperty = default)
+        {
+            return new global::Sample.Models.DerivedModel("derived", prop1, additionalBinaryDataProperties: null, stringProperty);
+        }
+
+        [global::System.ComponentModel.EditorBrowsableAttribute(global::System.ComponentModel.EditorBrowsableState.Never)]
+        public static global::Sample.Models.UnknownAbstractModel UnknownAbstractModel(string prop1, string kind)
+        {
+            return new global::Sample.Models.UnknownAbstractModel(kind, prop1, additionalBinaryDataProperties: null);
+        }
+    }
+}

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/ModelFactories/TestData/ModelFactoryProviderTests/BackCompatibility_UnknownDiscriminatorReturnTypeOverloadIsGenerated/SampleNamespaceModelFactory.cs

@@ -0,0 +1,18 @@
+using Sample.Models;
+
+namespace Sample.Namespace
+{
+    public static partial class SampleNamespaceModelFactory
+    {
+        // Previous contract shipped a factory whose return type was the Unknown* discriminator model itself,
+        // rather than the abstract base. It must be restored as a hidden back-compat overload.
+        public static UnknownAbstractModel UnknownAbstractModel(string prop1 = default, string kind = default)
+        { }
+    }
+}
+
+namespace Sample.Models
+{
+    public partial class UnknownAbstractModel
+    { }
+}