Middleware gives type error if using a function variable rather than defining inline

[callumgare]

Doing this works fine:

...
    .outputs([
      {
        status: 400,
        contentType: 'application/json',
        body: z.string(),
      },
    ])
    .middleware(async () => TypedNextResponse.json('test', { status: 400 }))
...

But doing this:

const middlewareFunction = async () => TypedNextResponse.json('test', { status: 400 });
...
    .outputs([
      {
        status: 400,
        contentType: 'application/json',
        body: z.string(),
      },
    ])
    .middleware(middlewareFunction)

Gives this error:

Argument of type '() => Promise<TypedNextResponseType<string, 400, AnyContentTypeWithAutocompleteForMostCommonOnes>>' is not assignable to parameter of type 'RouteMiddleware<BaseOptions, BaseOptions, unknown, number, AnyContentTypeWithAutocompleteForMostCommonOnes, { ...; }[], void | ... 1 more ... | NextResponse<...>>'.
  Type 'Promise<TypedNextResponseType<string, 400, AnyContentTypeWithAutocompleteForMostCommonOnes>>' is not assignable to type 'void | BaseOptions | TypedNextResponseType<string, 400, "application/json"> | NextResponse<string> | Promise<void | BaseOptions | TypedNextResponseType<...> | NextResponse<...>>'.
    Type 'Promise<TypedNextResponseType<string, 400, AnyContentTypeWithAutocompleteForMostCommonOnes>>' is not assignable to type 'Promise<void | BaseOptions | TypedNextResponseType<string, 400, "application/json"> | NextResponse<string>>'.
      Type 'TypedNextResponseType<string, 400, AnyContentTypeWithAutocompleteForMostCommonOnes>' is not assignable to type 'void | BaseOptions | TypedNextResponseType<string, 400, "application/json"> | NextResponse<string>'.
        Type 'TypedNextResponseType<string, 400, AnyContentTypeWithAutocompleteForMostCommonOnes>' is not assignable to type 'TypedNextResponseType<string, 400, "application/json">'.
          Type 'AnyContentTypeWithAutocompleteForMostCommonOnes' is not assignable to type '"application/json"'.
            Type 'string & Record<never, never>' is not assignable to type '"application/json"'.(2345)
const middlewareFunction: () => Promise<TypedNextResponseType<string, 400, AnyContentTypeWithAutocompleteForMostCommonOnes>>

I've made a reproducible stackblitz of the issue to demonstrate the issue.

Thanks for such a helpful framework btw!

---

I've narrowed it down slightly/come up with a workaround. This does work okay:

const middlewareFunction = async () => TypedNextResponse.json(
  'test', 
  { status: 400, headers: {'Content-Type': 'application/json'} }
);
...
    .outputs([
      {
        status: 400,
        contentType: 'application/json',
        body: z.string(),
      },
    ])
    .middleware(middlewareFunction)

I've updated the stackblitz to include this third example.