Fix review findings: extract isScalarLikeType, safe union type assertion

- Extract isScalarLikeType() to type-utils.ts shared utility, replacing
  duplicated inline check in union-type.tsx
- Replace unsafe `as Union` casts in union-type tests with assertUnionResult()
  helper that provides a clear error if the mutation returns a Model

Author: FionaBronwen

packages/graphql/src/components/types/union-type.tsx

@@ -1,20 +1,13 @@
-import { type Type, type Union, getDoc } from "@typespec/compiler";
+import { type Union, getDoc } from "@typespec/compiler";
 import * as gql from "@alloy-js/graphql";
 import { useTsp } from "@typespec/emitter-framework";
-import { getUnionName, toTypeName } from "../../lib/type-utils.js";
+import { getUnionName, isScalarLikeType, toTypeName } from "../../lib/type-utils.js";
 
 export interface UnionTypeProps {
   /** The union type to render */
   type: Union;
 }
 
-/**
- * Check if a type is a scalar (built-in or custom)
- */
-function isScalarType(type: Type): boolean {
-  return type.kind === "Scalar" || type.kind === "Intrinsic";
-}
-
 /**
  * Renders a GraphQL union type declaration
  * Scalars are wrapped in object types since GraphQL unions can only contain object types
@@ -32,7 +25,7 @@ export function UnionType(props: UnionTypeProps) {
     const variantName =
       typeof variant.name === "string" ? variant.name : String(variant.name);
 
-    if (isScalarType(variant.type)) {
+    if (isScalarLikeType(variant.type)) {
       // Reference the wrapper type for scalars (created by mutation engine)
       // Include union name to match wrapper model naming convention
       return toTypeName(name) + toTypeName(variantName) + "UnionVariant";

packages/graphql/src/lib/type-utils.ts

@@ -318,6 +318,11 @@ function getTemplateStringInternal(
   return args.length > 0 ? args.map(toTypeName).join(options.conjunction) : "";
 }
 
+/** Check if a type is a scalar (built-in or custom) or an intrinsic type like `unknown`. */
+export function isScalarLikeType(type: Type): boolean {
+  return type.kind === "Scalar" || type.kind === "Intrinsic";
+}
+
 /** Check if a model should be emitted as a GraphQL object type (not an array, record, or never). */
 export function isTrueModel(model: Model): boolean {
   return !(

packages/graphql/test/components/union-type.test.tsx

@@ -5,11 +5,22 @@ import { describe, expect, it, beforeEach } from "vitest";
 import { UnionType } from "../../src/components/types/index.js";
 import {
   createGraphQLMutationEngine,
+  type GraphQLUnionMutation,
   GraphQLTypeContext,
 } from "../../src/mutation-engine/index.js";
 import { Tester } from "../test-host.js";
 import { renderComponentToSDL } from "./component-test-utils.js";
 
+/** Assert that an output-context union mutation produced a Union (not a Model). */
+function assertUnionResult(mutation: GraphQLUnionMutation): Union {
+  if (mutation.mutatedType.kind !== "Union") {
+    throw new Error(
+      `Expected Union from output-context mutation, got ${mutation.mutatedType.kind}`,
+    );
+  }
+  return mutation.mutatedType;
+}
+
 describe("UnionType component", () => {
   let tester: Awaited<ReturnType<typeof Tester.createInstance>>;
   beforeEach(async () => {
@@ -27,6 +38,7 @@ describe("UnionType component", () => {
 
     const engine = createGraphQLMutationEngine(tester.program);
     const mutation = engine.mutateUnion(Pet, GraphQLTypeContext.Output);
+    const mutatedUnion = assertUnionResult(mutation);
 
     // Union members must be registered in the schema for buildSchema to resolve them
     const sdl = renderComponentToSDL(
@@ -38,7 +50,7 @@ describe("UnionType component", () => {
         <gql.ObjectType name="Dog">
           <gql.Field name="breed" type={gql.String} nonNull />
         </gql.ObjectType>
-        <UnionType type={mutation.mutatedType as Union} />
+        <UnionType type={mutatedUnion} />
       </>,
     );
 
@@ -59,6 +71,7 @@ describe("UnionType component", () => {
 
     const engine = createGraphQLMutationEngine(tester.program);
     const mutation = engine.mutateUnion(Result, GraphQLTypeContext.Output);
+    const mutatedUnion = assertUnionResult(mutation);
 
     const sdl = renderComponentToSDL(
       tester.program,
@@ -69,7 +82,7 @@ describe("UnionType component", () => {
         <gql.ObjectType name="Failure">
           <gql.Field name="message" type={gql.String} nonNull />
         </gql.ObjectType>
-        <UnionType type={mutation.mutatedType as Union} />
+        <UnionType type={mutatedUnion} />
       </>,
     );
 
@@ -89,6 +102,7 @@ describe("UnionType component", () => {
 
     const engine = createGraphQLMutationEngine(tester.program);
     const mutation = engine.mutateUnion(Shape, GraphQLTypeContext.Output);
+    const mutatedUnion = assertUnionResult(mutation);
 
     const sdl = renderComponentToSDL(
       tester.program,
@@ -102,7 +116,7 @@ describe("UnionType component", () => {
         <gql.ObjectType name="Triangle">
           <gql.Field name="base" type={gql.Float} nonNull />
         </gql.ObjectType>
-        <UnionType type={mutation.mutatedType as Union} />
+        <UnionType type={mutatedUnion} />
       </>,
     );
 
@@ -122,6 +136,7 @@ describe("UnionType component", () => {
 
     const engine = createGraphQLMutationEngine(tester.program);
     const mutation = engine.mutateUnion(Mixed, GraphQLTypeContext.Output);
+    const mutatedUnion = assertUnionResult(mutation);
 
     // Register Cat and the wrapper type that the union will reference
     const sdl = renderComponentToSDL(
@@ -133,7 +148,7 @@ describe("UnionType component", () => {
         <gql.ObjectType name="MixedTextUnionVariant">
           <gql.Field name="value" type={gql.String} nonNull />
         </gql.ObjectType>
-        <UnionType type={mutation.mutatedType as Union} />
+        <UnionType type={mutatedUnion} />
       </>,
     );