Failed to statically analyse src/routes/ Unexpected token error for loader with async generic function

[minht11]

Describe the bug

After #13684 I get Failed to statically analyse when I have additional function with generics:

// import {} from './config'

const loadDataWithGeneric = async <T>() => {};

export const load = () => {};

Comment is needed as well, to trigger it.

Reproduction

Reproduction

Logs

SyntaxError: Failed to statically analyse src/routes/(app)/+layout.ts. Unexpected token (3:40)
...
acorn/dist/acorn.mjs:596:15) {
  pos: 70,
  loc: Position { line: 3, column: 40 },
  raisedAt: 72
}

System Info

npmPackages:
    @sveltejs/adapter-static: ^3.0.8 => 3.0.8 
    @sveltejs/kit: ^2.21.1 => 2.21.1 
    @sveltejs/vite-plugin-svelte: 5.0.3 => 5.0.3 
    rolldown-vite:  6.3.10 
    svelte: ^5.31.1 => 5.31.1

Severity

blocking an upgrade

Additional Information

No response

[vteslenko]

I'm experiencing a similar issue that prevents upgrading past kit v2.20.8

Maybe the TS file was evaluated as JS because it breaks on TS-specific language constructs.
As an example, imagine a +page.ts file:

Breaks:

export async function load(event: PageLoadEvent) {
    return await someWrapperFunction(event, async (session) => {
        //logic
        return {/*...result...*/};
    });
}

Works:

export async function load(event) {
    async function process(session){
        //logic
        return {/*...result...*/};
    }

    return await someWrapperFunction(event, process);
}

The error for the first file would be:

SyntaxError: Failed to statically analyse src/routes/(website)/dashboard/+page.ts. Unexpected token (9:34)
    at pp$4.raise (/node_modules/@sveltejs/kit/node_modules/acorn/dist/acorn.mjs:3731:13)
    at TypeScriptParser.raiseCommonCheck (/node_modules/@sveltejs/kit/node_modules/@sveltejs/acorn-typescript/index.js:4879:36)
    at TypeScriptParser.raise (/node_modules/@sveltejs/kit/node_modules/@sveltejs/acorn-typescript/index.js:4885:21) 
    at pp$9.unexpected (/node_modules/@sveltejs/kit/node_modules/acorn/dist/acorn.mjs:770:8)
    at TypeScriptParser.tsParseNonArrayType (/node_modules/@sveltejs/kit/node_modules/@sveltejs/acorn-typescript/index.js:2382:14)
    at TypeScriptParser.tsParseArrayTypeOrHigher (/node_modules/@sveltejs/kit/node_modules/@sveltejs/acorn-typescript/index.js:2385:25)
    at /node_modules/@sveltejs/kit/node_modules/@sveltejs/acorn-typescript/index.js:2404:166
    at TypeScriptParser.tsInAllowConditionalTypesContext (/node_modules/@sveltejs/kit/node_modules/@sveltejs/acorn-typescript/index.js:1959:18)
    at TypeScriptParser.tsParseTypeOperatorOrHigher (/node_modules/@sveltejs/kit/node_modules/@sveltejs/acorn-typescript/index.js:2404:122)
    at TypeScriptParser.tsParseUnionOrIntersectionType (/node_modules/@sveltejs/kit/node_modules/@sveltejs/acorn-typescript/index.js:2117:22) {
  pos: 319,
  loc: Position { line: 9, column: 34 },
  raisedAt: 332
}