I spent a bit of time to realize that this refinement type won't cause any reflective call, and is only needed to narrow down a member type. It would be better to use `override` in this case, even if not required by scalac. I leave below my initial reaction to this code (followed by building scala again with `-feature` :)).

Since this is a refinement type, calls to this member will be reflective. It might introduce subtle slowdowns (symbol loaders are on the hot path).