Ok, so I'm trying to implement a simple "Command Bus" in TypeScript, but I'm tripping up over generics and I wonder if someone could help me. Here is my code:

This is the interface for the commandbus

export default interface CommandBus {
  execute: <C extends Command, R extends Response<C>>(command: C) => Promise<R>;
}

This is the implementation

export default class AppCommandBus implements CommandBus {
  private readonly handlers: Handler<Command, Response<Command>>[];

  /* ... constructor ... */

  public async execute<C extends Command, R extends Response<C>>(
    command: C
  ): Promise<R> {
    const resolvedHandler = this.handlers.find(handler =>
      handler.canHandle(command)
    );

    /* ... check if undef and throw ... */

    return resolvedHandler.handle(command);
  }
}

and this is what the Handler interface looks like:

export default interface Handler<C extends Command, R extends Response<C>> {
  canHandle: (command: C) => boolean;
  handle: (command: C) => Promise<R>;
}

Command is (currently) an empty interface, and Response looks like this:

export default interface Response<C extends Command> {
  command: C;
}

I'm getting the follow compile error error against the last line of the execute function of the commandbus and I'm completely stumped.

type 'Response<Command>' is not assignable to type 'R'. 'R' could be instantiated with an arbitrary type which could be unrelated to 'Response<Command>'.

If anyone is able to help me understand what I'm doing wrong, I'd be eternally grateful!

EDIT

I've realised I can work around this with a typecast:

const resolvedHandler = (this.handlers.find(handler =>
  handler.canHandle(command)
) as unknown) as Handler<C, R> | undefined;

But I'd still like to know how to resolve this double cast.