[http-client-csharp] Preserve last-contract model base types

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

@@ -46,6 +46,7 @@ public class ScmModelProvider : ModelProvider
 
         internal const string ScmEvaluationTypeDiagnosticId = "SCME0001";
         internal const string FileBinaryContentDiagnosticId = "SCME0004";
+        private const string IncompatibleBackcompatBaseTypeDiagnostic = "incompatible-backcompat-base-type";
 
         internal const string ScmEvaluationTypeSuppressionJustification =
             "Type is for evaluation purposes only and is subject to change or removal in future updates.";
@@ -73,6 +74,22 @@ public ScmModelProvider(InputModelType inputModel) : base(inputModel)
             BaseJsonPatchProperty = new(GetBaseJsonPatchProperty());
         }
 
+        protected override CSharpType? BuildBaseTypeForBackCompatibility(CSharpType? currentBase)
+        {
+            var previousBase = LastContractView?.BaseType;
+            if (_inputModel.DiscriminatorValue is not null &&
+                previousBase is not null &&
+                !IsInBaseTypeHierarchy(currentBase, previousBase))
+            {
+                CodeModelGenerator.Instance.Emitter.ReportDiagnostic(
+                    IncompatibleBackcompatBaseTypeDiagnostic,
+                    $"Could not preserve base type '{previousBase.FullyQualifiedName}' on model '{BuildNamespace()}.{BuildName()}' because the model participates in the current discriminator hierarchy.");
+                return currentBase;
+            }
+
+            return base.BuildBaseTypeForBackCompatibility(currentBase);
+        }
+
         protected override FieldProvider[] BuildFields()
         {
             if (JsonPatchField is null)

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/ScmModelProvider/ScmModelProviderTests.cs

@@ -1,11 +1,13 @@
 // Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT License.
 
+using System;
 using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Text.Json.Serialization;
 using System.Threading.Tasks;
+using Microsoft.CodeAnalysis;
 using Microsoft.TypeSpec.Generator.Expressions;
 using Microsoft.TypeSpec.Generator.Input;
 using Microsoft.TypeSpec.Generator.Primitives;
@@ -262,6 +264,70 @@ await MockHelpers.LoadMockGeneratorAsync(
             Assert.AreEqual(Helpers.GetExpectedFromFile("Serialization"), serializationContent);
         }
 
+        [Test]
+        public async Task BackCompat_DifferentLastContractBaseIsNotRestoredAcrossDiscriminatorHierarchy()
+        {
+            var previousBase = InputFactory.Model("previousBase", properties: []);
+            var derivedModel = InputFactory.Model(
+                "derivedModel",
+                discriminatedKind: "derived",
+                usage: InputModelTypeUsage.Json,
+                properties: []);
+            var currentBase = InputFactory.Model(
+                "currentBase",
+                usage: InputModelTypeUsage.Json,
+                properties:
+                [
+                    InputFactory.Property("kind", InputPrimitiveType.String, isRequired: true, isDiscriminator: true)
+                ],
+                discriminatedModels: new Dictionary<string, InputModelType> { ["derived"] = derivedModel });
+
+            await MockHelpers.LoadMockGeneratorAsync(
+                lastContractCompilation: async () => await Helpers.GetCompilationFromDirectoryAsync(),
+                inputModels: () => [previousBase, currentBase, derivedModel]);
+
+            var models = ScmCodeModelGenerator.Instance.OutputLibrary.TypeProviders
+                .OfType<ScmModel>()
+                .ToArray();
+            foreach (var model in models)
+            {
+                model.ProcessTypeForBackCompatibility();
+            }
+
+            var derivedProvider = models.Single(t => t.Name == "DerivedModel");
+            Assert.Multiple(() =>
+            {
+                Assert.AreEqual("PreviousBase", derivedProvider.LastContractView?.BaseType?.Name,
+                    "The regression requires a different last-contract base");
+                Assert.AreEqual("CurrentBase", derivedProvider.BaseType?.Name,
+                    "The current discriminator hierarchy must remain assignable");
+                Assert.That(models, Has.Some.Matches<ScmModel>(m => m.IsUnknownDiscriminatorModel),
+                    "The current discriminator hierarchy should include its unknown subtype");
+            });
+
+            string[] supportingProviderNames = ["ModelSerializationExtensions", "ChangeTrackingDictionary", "SampleContext", "TypeFormatters", "SerializationFormat"];
+            var generatedProviders = ScmCodeModelGenerator.Instance.OutputLibrary.TypeProviders
+                .Where(provider => provider is ScmModel || supportingProviderNames.Contains(provider.Name))
+                .Concat(models.SelectMany(model => model.SerializationProviders))
+                .Distinct()
+                .ToArray();
+            var sourceFiles = generatedProviders
+                .Select(provider => (
+                    Name: $"{provider.Name}.cs",
+                    Content: new TypeProviderWriter(provider).Write().Content))
+                .Append((
+                    Name: "SampleContext.Default.cs",
+                    Content: "namespace Sample { public partial class SampleContext { public static SampleContext Default => null; } }"))
+                .Append((
+                    Name: "SampleTypeSpecContext.Default.cs",
+                    Content: "namespace SampleTypeSpec { public partial class SampleTypeSpecContext : System.ClientModel.Primitives.ModelReaderWriterContext { public static SampleTypeSpecContext Default => null; } }"));
+            var compilation = await Helpers.GetCompilationFromSourceFilesAsync(sourceFiles);
+            Assert.That(
+                compilation.GetDiagnostics().Where(d => d.Severity is DiagnosticSeverity.Warning or DiagnosticSeverity.Error),
+                Is.Empty,
+                "The discriminator deserializer should compile after incompatible base restoration is skipped");
+        }
+
         [Test]
         public async Task BackCompat_AccessibleParameterlessSerializationConstructorIsPreserved()
         {

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/ScmModelProvider/TestData/ScmModelProviderTests/BackCompat_DifferentLastContractBaseIsNotRestoredAcrossDiscriminatorHierarchy/Models.cs

@@ -0,0 +1,10 @@
+namespace Sample.Models
+{
+    public partial class PreviousBase
+    {
+    }
+
+    public partial class DerivedModel : PreviousBase
+    {
+    }
+}

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

@@ -22,6 +22,9 @@ public enum BackCompatibilityChangeCategory
         /// <summary>A property type was preserved from the last contract.</summary>
         PropertyTypePreserved,
 
+        /// <summary>A model base type was preserved from the last contract.</summary>
+        ModelBaseTypePreserved,
+
         /// <summary>A constructor modifier (e.g. <c>private protected</c> -&gt; <c>public</c>) was preserved from the last contract.</summary>
         ConstructorModifierPreserved,
 

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

@@ -174,6 +174,7 @@ public void WriteBufferedMessages()
             BackCompatibilityChangeCategory.ParameterNamePreserved => "Parameter Name Preserved",
             BackCompatibilityChangeCategory.AdditionalPropertiesShapePreserved => "AdditionalProperties Shape Preserved",
             BackCompatibilityChangeCategory.PropertyTypePreserved => "Property Type Preserved",
+            BackCompatibilityChangeCategory.ModelBaseTypePreserved => "Model Base Type Preserved",
             BackCompatibilityChangeCategory.ConstructorModifierPreserved => "Constructor Modifier Preserved",
             BackCompatibilityChangeCategory.EnumMemberReordering => "Enum Member Reordering",
             BackCompatibilityChangeCategory.ApiVersionEnumMemberAdded => "Api Version Enum Member Added From Last Contract",