Add a discriminated union sample (#919)

packages/playground/e2e/ui.e2e.ts

@@ -6,7 +6,7 @@ const ctrlOrCmd = process.platform === "darwin" ? "Meta" : "Control";
 test("compiled http sample", async ({ page }) => {
   await page.goto(host);
   const samplesDropDown = page.locator("select.sample-dropdown");
-  await samplesDropDown.selectOption({ label: "Http" });
+  await samplesDropDown.selectOption({ label: "Http service" });
   const outputContainer = page.locator(".output-content");
   await expect(outputContainer).toContainText(`"title": "Widget Service"`);
 });

packages/playground/samples/unions.cadl

@@ -0,0 +1,42 @@
+import "@cadl-lang/rest";
+import "@cadl-lang/openapi3";
+
+@serviceTitle("Widget Service")
+namespace DemoService;
+using Cadl.Rest;
+using Cadl.Http;
+using OpenAPI;
+
+model WidgetBase {
+  @key id: string;
+  weight: int32;
+  color: "red" | "blue";
+}
+
+enum WidgetKind {
+  Heavy,
+  Light,
+}
+
+model HeavyWidget extends WidgetBase {
+  kind: WidgetKind.Heavy;
+}
+
+model LightWidget extends WidgetBase {
+  kind: WidgetKind.Light;
+}
+
+@discriminator("kind")
+@oneOf
+union Widget {
+  heavy: HeavyWidget,
+  light: LightWidget,
+}
+
+@error
+model Error {
+  code: int32;
+  message: string;
+}
+
+@get op read(@path id: string): Widget | Error;

packages/playground/vite.config.ts

@@ -10,9 +10,10 @@ export default definePlaygroundViteConfig({
     "@cadl-lang/openapi3",
   ],
   samples: {
-    Http: "samples/http.cadl",
-    "Rest framework": "samples/rest.cadl",
-    "Versioned Rest framework": "samples/versioning.cadl",
+    "API versioning": "samples/versioning.cadl",
+    "Discriminated unions": "samples/unions.cadl",
+    "Http service": "samples/http.cadl",
+    "REST framework": "samples/rest.cadl",
   },
   enableSwaggerUI: true,
 });