Use Statically Type Links type as a return type

[dawnchan030920]

Statically typed links are cool, but I ran into issues when trying to use Router<T> as the return type of a function. Only static route strings are acceptable, dynamic route strings will cause type error.
I checked the link.d.ts and found that corresponding dynamic route string types are generated.
link.d.ts

declare namespace __next_route_internal_types__ {
  type SearchOrHash = `?${string}` | `#${string}`
  type WithProtocol = `${string}:${string}`

  type Suffix = '' | SearchOrHash

  type SafeSlug<S extends string> = S extends `${string}/${string}`
    ? never
    : S extends `${string}${SearchOrHash}`
    ? never
    : S extends ''
    ? never
    : S

  type CatchAllSlug<S extends string> = S extends `${string}${SearchOrHash}`
    ? never
    : S extends ''
    ? never
    : S

  type OptionalCatchAllSlug<S extends string> =
    S extends `${string}${SearchOrHash}` ? never : S

  type StaticRoutes = 
    | `/`
    | `/projects`
    | `/tags`
  type DynamicRoutes<T extends string = string> = 
    | `/category/${SafeSlug<T>}`
    | `/series/${SafeSlug<T>}`
    | `/posts/${SafeSlug<T>}`
    | `/projects/${SafeSlug<T>}`
    | `/tags/${SafeSlug<T>}`

  type RouteImpl<T> = 
    | StaticRoutes
    | SearchOrHash
    | WithProtocol
    | `${StaticRoutes}${SearchOrHash}`
    | (T extends `${DynamicRoutes<infer _>}${Suffix}` ? T : never)
    
}

function I am trying to build

const FilterItemToRoute = <T extends string>(
  item: FilterItem
): Route<T> | URL =>
  match(item.condition)
    .with("category", () => `/category/${item.value}` as const)
    .with("series", () => `/series/${item.value}` as const)
    .exhaustive();

If I change /category/${item.value} into something like / or /projects which are valid static route strings, the compiler won't complain.