Use allof when building nullable enum (#4727)

fix [#4398](#4398)

.chronus/changes/fix-openapi3-nullable-enum-allof-2024-9-14-20-57-59.md

@@ -0,0 +1,8 @@
+---
+# Change versionKind to one of: internal, fix, dependencies, feature, deprecation, breaking
+changeKind: fix
+packages:
+  - "@typespec/openapi3"
+---
+
+Nullable enum use `allOf` instead of `oneOf`

packages/openapi3/src/schema-emitter.ts

@@ -576,7 +576,7 @@ export class OpenAPI3SchemaEmitter extends TypeEmitter<
               ...additionalProps,
             });
           } else {
-            return new ObjectBuilder({ oneOf: B.array([schema]), ...additionalProps });
+            return new ObjectBuilder({ allOf: B.array([schema]), ...additionalProps });
           }
         } else {
           const merged = new ObjectBuilder<OpenAPI3Schema>(schema);

packages/openapi3/test/nullable-properties.test.ts

@@ -34,7 +34,7 @@ describe("openapi3: nullable properties", () => {
         }
         `,
     );
-    deepStrictEqual(res.schemas.X.properties.prop.oneOf, [
+    deepStrictEqual(res.schemas.X.properties.prop.allOf, [
       {
         $ref: "#/components/schemas/A",
       },

packages/openapi3/test/union-schema.test.ts

@@ -1,6 +1,6 @@
 import { expectDiagnostics } from "@typespec/compiler/testing";
 import { deepStrictEqual, ok, strictEqual } from "assert";
-import { describe, it } from "vitest";
+import { describe, expect, it } from "vitest";
 import { diagnoseOpenApiFor, oapiForModel, openApiFor } from "./test-host.js";
 
 describe("openapi3: union type", () => {
@@ -577,4 +577,21 @@ describe("openapi3: union type", () => {
       },
     });
   });
+
+  describe("null and another single variant produce allOf", () => {
+    it.each([
+      ["model", "model Other {}"],
+      ["enum", "enum Other {a, b}"],
+    ])("%s variant", async (_, code) => {
+      const openApi = await openApiFor(`
+        union Test { Other, null }
+        ${code}
+        `);
+
+      expect(openApi.components.schemas.Test).toMatchObject({
+        allOf: [{ $ref: "#/components/schemas/Other" }],
+        nullable: true,
+      });
+    });
+  });
 });