Prevent C# model factories from instantiating abstract unknown variants

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

@@ -722,7 +722,8 @@ private static IReadOnlyList<ValueExpression> GetCtorArgs(
             }
 
             return modelProvider.DeclarationModifiers.HasFlag(TypeSignatureModifiers.Abstract)
-                ? modelProvider.DerivedModels.FirstOrDefault(m => m.IsUnknownDiscriminatorModel)
+                ? modelProvider.DerivedModels.FirstOrDefault(m => m.IsUnknownDiscriminatorModel
+                    && !m.DeclarationModifiers.HasFlag(TypeSignatureModifiers.Abstract))
                 : modelProvider;
         }
 

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

@@ -1821,6 +1821,59 @@ public async Task BackCompatibility_AbstractReturnTypeOverloadIsGenerated()
             Assert.AreEqual(Helpers.GetExpectedFromFile(), content);
         }
 
+        [Test]
+        public async Task BackCompatibility_AbstractReturnTypeWithoutCurrentOverload()
+        {
+            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);
+        }
+
+        [TestCase(false)]
+        [TestCase(true)]
+        public async Task BackCompatibility_AbstractReturnTypeWithoutConcreteUnknownModelIsSkipped(bool hasUnknownModel)
+        {
+            var derived = InputFactory.Model("DerivedModel", discriminatedKind: "derived");
+            var baseModel = InputFactory.Model(
+                "AbstractModel",
+                properties:
+                [
+                    InputFactory.Property("kind", InputPrimitiveType.String, isRequired: true, isDiscriminator: hasUnknownModel),
+                    InputFactory.Property("prop1", InputPrimitiveType.String),
+                ],
+                derivedModels: hasUnknownModel ? [derived] : []);
+
+            _instance = (await MockHelpers.LoadMockGeneratorAsync(
+                inputNamespaceName: "Sample.Namespace",
+                inputModelTypes: hasUnknownModel ? [baseModel, derived] : [baseModel],
+                compilation: async () => await Helpers.GetCompilationFromDirectoryAsync(),
+                lastContractCompilation: async () => await Helpers.GetCompilationFromDirectoryAsync(
+                    method: nameof(BackCompatibility_AbstractReturnTypeWithoutCurrentOverload)))).Object;
+
+            var modelFactory = _instance!.OutputLibrary.ModelFactory.Value;
+            modelFactory.ProcessTypeForBackCompatibility();
+
+            Assert.IsEmpty(modelFactory.Methods.Where(m => m.Signature.Name is "AbstractModel" or "AbstractModelOldName"));
+        }
+
         [Test]
         public async Task BackCompatibility_UnknownDiscriminatorReturnTypeOverloadIsGenerated()
         {

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

@@ -0,0 +1,8 @@
+namespace Sample.Models
+{
+    public abstract partial class AbstractModel
+    { }
+
+    internal abstract partial class UnknownAbstractModel : AbstractModel
+    { }
+}

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/ModelFactories/TestData/ModelFactoryProviderTests/BackCompatibility_AbstractReturnTypeWithoutCurrentOverload.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 AbstractModelOldName(string prop1, string kind)
+        {
+            return new global::Sample.Models.UnknownAbstractModel(kind, prop1, default, additionalBinaryDataProperties: null);
+        }
+    }
+}

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

@@ -0,0 +1,16 @@
+using Sample.Models;
+
+namespace Sample.Namespace
+{
+    public static partial class SampleNamespaceModelFactory
+    {
+        public static AbstractModel AbstractModelOldName(string prop1 = default, string kind = default)
+        { }
+    }
+}
+
+namespace Sample.Models
+{
+    public abstract partial class AbstractModel
+    { }
+}