Passing a trait as a parameter to `Dict`?

[flint cloud]

This is related to a proposal for adding Middleware functionality to Lightbug HTTP: https://github.com/saviorand/lightbug_http/pull/33#issuecomment-2105547981
We're trying to do something like the following:

## HTTPHandler is an interface for handling HTTP requests in the RouterMiddleware.
## It is a leaf node in the middleware chain.
trait HTTPHandler:
    fn handle(self, context: Context) -> HTTPResponse:
        ...

## Router middleware routes requests to different middleware based on the path.
@value
struct RouterMiddleware(Middleware):
    var next: Middleware
    var routes: Dict[String, HTTPHandler]

Where routes: Dict[String, HTTPHandler] takes in an HTTPHandler trait which can be implemented by different handlers, including ones created by the user.
Currently, since Dict expects a value that implements CollectionElement, so we're getting the following error:

middleware/router.mojo:15:36: error: 'Dict' parameter #1 has 'CollectionElement' type, but value has type 'AnyTrait[HTTPHandler]'
        self.routes = Dict[String, HTTPHandler]()

This persists even if HTTPHandler extends CollectionElement like so:
trait HTTPHandler(CollectionElement).

Any ideas on how we can get this to work?

[uncut adder]

To help others find answers, you can mark your question as solved via Right click solution message -> Apps -> ✅ Mark Solution

[polar kestrel]

I think you can do the following:

@value
struct RouterMiddleware[HTTPHandlerType: HTTPHandler](Middleware):
    var next: Middleware
    var routes: Dict[String, HTTPHandlerType]