Since Scrooge removed the higher-kinded service traits implemented in

com.twitter.util.Future

, and inlined those method definitions into MethodPerEndpoint traits, we had to reintroduce the higher-kinded traits with our AddCatsTaglessInstances scalafix rule.

The problem is that Finagle uses reflection to figure out what class to instantiate, and the reflection logic specifically looks for ServiceName$$MethodPerEndpoint, where ServiceName is the

com.twitter.finagle.thrift.GeneratedThriftService

object. After running the Scalafix rule, the ServiceName$$MethodPerEndpoint definition will extend ServiceName[Future], but passing an instance of ServiceName[Future] isn't good enough: it seems like it must be the MethodPerEndpoint.

This typeclass encodes the relationship between the higher-kinded service trait and its MethodPerEndpoint counterpart, so that if we have an instance of the service trait, we can convert to and from the related MethodPerEndpoint.