[TCGC] Convert inline discriminated unions to polymorphic models by default

.chronus/changes/tcgc-inline-discriminated-union-conversion.md

@@ -0,0 +1,7 @@
+---
+changeKind: feature
+packages:
+  - "@azure-tools/typespec-client-generator-core"
+---
+
+Convert inline-discriminated unions with explicit nominal model hierarchies to polymorphic SDK models by default. Set `convertDiscriminatedUnionsToModels: false` in `createSdkContext` to preserve union representations. Ordinary unions and object-envelope unions retain their existing behavior.

packages/typespec-client-generator-core/src/context.ts

@@ -71,6 +71,7 @@ export function createTCGCContext(
 
     previewStringRegex: /-preview$/,
     disableUsageAccessPropagationToBase: false,
+    convertDiscriminatedUnionsToModels: true,
     generateProtocolMethods: true,
     generateConvenienceMethods: true,
     __referencedTypeCache: new Map<
@@ -173,6 +174,8 @@ export interface CreateSdkContextOptions {
   exportTCGCoutput?: boolean; // this flag is for emitter to export TCGC output as yaml file
   flattenUnionAsEnum?: boolean; // this flag is for emitter to decide whether tcgc should flatten union as enum
   enableLegacyHierarchyBuilding?: boolean; // this flag is for emitter to decide whether tcgc should respect the `@hierarchyBuilding` decorator
+  /** Convert eligible inline-discriminated unions to model hierarchies. Default: true. */
+  convertDiscriminatedUnionsToModels?: boolean;
 }
 
 export async function createSdkContext<
@@ -238,6 +241,7 @@ export async function createSdkContext<
     disableUsageAccessPropagationToBase: options?.disableUsageAccessPropagationToBase ?? false,
     flattenUnionAsEnum: options?.flattenUnionAsEnum ?? true,
     enableLegacyHierarchyBuilding: options?.enableLegacyHierarchyBuilding ?? true,
+    convertDiscriminatedUnionsToModels: options?.convertDiscriminatedUnionsToModels ?? true,
   };
 
   if (context.options["examples-dir"]) {

packages/typespec-client-generator-core/src/decorators.ts

@@ -712,7 +712,12 @@ export function getUsage(context: TCGCContext, entity: Model | Enum | Union): Us
   switch (entity.kind) {
     case "Union":
       const type = getSdkUnion(context, entity);
-      if (type.kind === "enum" || type.kind === "union" || type.kind === "nullable") {
+      if (
+        type.kind === "model" ||
+        type.kind === "enum" ||
+        type.kind === "union" ||
+        type.kind === "nullable"
+      ) {
         return type.usage;
       }
       return UsageFlags.None;
@@ -771,7 +776,12 @@ export function getAccess(
       return getSdkEnum(context, entity).access;
     case "Union": {
       const type = getSdkUnion(context, entity);
-      if (type.kind === "enum" || type.kind === "union" || type.kind === "nullable") {
+      if (
+        type.kind === "model" ||
+        type.kind === "enum" ||
+        type.kind === "union" ||
+        type.kind === "nullable"
+      ) {
         return type.access;
       }
       return "public";

packages/typespec-client-generator-core/src/diagnostics/union-model-conversion-conflict.md

@@ -0,0 +1,72 @@
+This diagnostic is issued when an inline-discriminated union's discriminator property or tag mappings conflict with another union or an existing model discriminator in the same hierarchy.
+
+## Impact
+
+- **Area:** SDK union type conversion. The union resolves to its declared base SDK model, but incompatible discriminator metadata prevents generating a valid polymorphic hierarchy. TCGC reports an error instead of overwriting existing discriminator metadata.
+- **Not affected:** The TypeSpec union declaration, service routes, and wire payloads are unchanged.
+
+## ❌ Incorrect Usage
+
+```typespec
+@service
+namespace Example;
+
+model PetBase {
+  name: string;
+}
+
+model Cat extends PetBase {
+  meows: boolean;
+}
+
+@discriminated(#{ envelope: "none" })
+union Pet extends PetBase {
+  cat: Cat,
+  feline: Cat, // one model cannot have two polymorphic discriminator values
+}
+
+op get(): Pet;
+```
+
+## Diagnostic Message
+
+TCGC reports:
+
+```text
+Discriminated union 'Pet' has conflicting discriminator metadata for its base model hierarchy. Use consistent discriminator properties and tag mappings, or set `convertDiscriminatedUnionsToModels` to `false` to retain union representations.
+```
+
+## ✅ How to Fix
+
+Use consistent discriminator properties and tag mappings throughout the shared hierarchy. Give each alternative with a different wire tag its own model.
+
+```typespec
+@service
+namespace Example;
+
+model PetBase {
+  name: string;
+}
+
+model Cat extends PetBase {
+  meows: boolean;
+}
+
+model Dog extends PetBase {
+  barks: boolean;
+}
+
+@discriminated(#{ envelope: "none" })
+union Pet extends PetBase {
+  cat: Cat,
+  dog: Dog,
+}
+
+op get(): Pet;
+```
+
+Emitters that intentionally use native discriminated unions can set `convertDiscriminatedUnionsToModels: false` in `createSdkContext` instead of requesting polymorphic conversion.
+
+## Suppression
+
+This diagnostic should not be suppressed. Align the hierarchy's discriminator metadata, or disable conversion when the emitter intentionally retains discriminated unions.

packages/typespec-client-generator-core/src/interfaces.ts

@@ -66,7 +66,9 @@ export interface TCGCContext {
   disableUsageAccessPropagationToBase: boolean;
   flattenUnionAsEnum?: boolean;
   enableLegacyHierarchyBuilding?: boolean;
+  convertDiscriminatedUnionsToModels?: boolean;
 
+  __discriminatedUnionModels?: Map<Union, Model>;
   __referencedTypeCache: Map<Type, SdkModelType | SdkEnumType | SdkUnionType | SdkNullableType>;
   __arrayDictionaryCache: Map<Type, SdkDictionaryType | SdkArrayType>;
   __methodParameterCache: Map<ModelProperty, SdkMethodParameter>;

packages/typespec-client-generator-core/src/lib.ts

@@ -171,6 +171,13 @@ function doc(code: string) {
 export const $lib = createTypeSpecLibrary({
   name: "@azure-tools/typespec-client-generator-core",
   diagnostics: {
+    "union-model-conversion-conflict": {
+      ...doc("union-model-conversion-conflict"),
+      severity: "error",
+      messages: {
+        default: paramMessage`Discriminated union '${"unionName"}' has conflicting discriminator metadata for its base model hierarchy. Use consistent discriminator properties and tag mappings, or set \`convertDiscriminatedUnionsToModels\` to \`false\` to retain union representations.`,
+      },
+    },
     "union-null": {
       ...doc("union-null"),
       severity: "warning",