fix: use Promise<Response> for async kit handler return types

.changeset/forty-tools-cheat.md

@@ -0,0 +1,6 @@
+---
+'svelte-check': patch
+'svelte2tsx': patch
+---
+
+fix: use Promise<Response> for async kit handler return types

packages/svelte-check/test-success/src/routes/$types.d.ts

@@ -1 +1,6 @@
 export type PageData = {};
+export type RequestEvent = {
+    url: URL;
+    request: Request;
+    params: Record<string, string>;
+};

packages/svelte-check/test-success/src/routes/+server.ts

@@ -0,0 +1,8 @@
+export async function GET({ url }) {
+    return new Response(url.pathname);
+}
+
+export async function POST({ request }) {
+    const body = await request.text();
+    return new Response(body, { status: 201 });
+}

packages/svelte2tsx/src/helpers/sveltekit.ts

@@ -485,11 +485,23 @@ function addTypeToFunction(
             const paramInsertion = surround(!returnType ? `: Parameters<${type}>[0]` : `: ${type}`);
             insert(paramPos, paramInsertion);
             if (!fn.node.type && fn.node.body) {
+                const isAsync =
+                    fn.node.modifiers?.some((m) => m.kind === ts.SyntaxKind.AsyncKeyword) ?? false;
+                let effectiveReturnType = returnType;
+                if (isAsync && returnType) {
+                    // Async functions must return Promise<T>, not T | Promise<T>
+                    const promisePart = returnType
+                        .split(' | ')
+                        .find((t) => t.startsWith('Promise<'));
+                    if (promisePart) {
+                        effectiveReturnType = promisePart;
+                    }
+                }
                 const returnPos = ts.isArrowFunction(fn.node)
                     ? fn.node.equalsGreaterThanToken.getStart()
                     : fn.node.body.getStart();
                 const returnInsertion = surround(
-                    !returnType ? `: ReturnType<${type}> ` : `: ${returnType} `
+                    !effectiveReturnType ? `: ReturnType<${type}> ` : `: ${effectiveReturnType} `
                 );
                 insert(returnPos, returnInsertion);
             }

packages/svelte2tsx/test/helpers/index.ts

@@ -71,7 +71,15 @@ describe('Internal Helpers - upsertKitFile', () => {
         upsert(
             '+server.ts',
             `export async function GET(e) {}`,
-            `export async function GET(e: import('./$types.js').RequestEvent) : Response | Promise<Response> {}`
+            `export async function GET(e: import('./$types.js').RequestEvent) : Promise<Response> {}`
+        );
+    });
+
+    it('upserts GET non-async function', () => {
+        upsert(
+            '+server.ts',
+            `export function GET(e) { return new Response(); }`,
+            `export function GET(e: import('./$types.js').RequestEvent) : Response | Promise<Response> { return new Response(); }`
         );
     });