Using a scoped polymorphic type for react-router bindings to loader function

I’ve just also run into a similar problem as Error: Field value has a type which is less general than the (defined) scoped polymorphic type writing bindings for react-router v6:

This type definition for a RouteObject:

type rec t = {
  caseSensitive?: bool,
  children?: array<t>,
  element?: React.element,
  index?: bool,
  path?: string,
  loader?: 'data. unit => promise<'data>,
  action?: unknown, // TODO
  errorElement?: React.element,
  hydrateFallbackElement?: React.element,
  @as("Component") component?: React.component<{}>,
  @as("ErrorBoundary") errorBoundary?: React.component<{}>,
  // handle?: unknown
  shouldRevalidate?: bool,
  @as("lazy") lazy_?: unit => promise<unknown>,
}

When actually attempting to use it gives this type error

  60 │   caseSensitive: true,
  61 │   element: <Settings />,
  62 │   loader: async (type data, ()) => {
  63 │     let response = await Fetch.fetch("/api/settings", {method: #GET})
   . │ ...
  66 │     json
  67 │   },
  68 │ }
  69 │ 

  This has type: unit => promise<Js.Json.t>
  But it's expected to have type: 'data. unit => promise<'data>

Now I could reach for the GADT-style of binding the route objects, and in some respects that makes sense for the union type. But does anyone know how you could type-annotate this to use this feature? I can imagine a good number of Javascript APIs that’d become a lot less verbose to use with this–presuming you don’t have to use @send bindings to get polymorphic behavior on a record type with a function.