• 您可以尝试match types

trait Entity:
  type Key

object Entity:
  type Aux[K] = Entity { type Key = K }

// match type
type EntityKey[T <: Entity] = T match
  case Entity.Aux[k] => k // k being lower-case is significant

type Dictionary[T <: Entity] = Map[EntityKey[T], T]

我不得不引入Aux-type，因为匹配类型似乎不适用于细化类型

type EntityKey[T <: Entity] = T match
  case Entity { type Key = k } => k // Not found: type k

请注意，在值级别上匹配类型可能不如类型投影工作得好。

• 除了匹配类型之外，type classes也可以是一般类型投影的替代

trait Entity:
  type Key

// type class
trait EntityKey[T <: Entity]:
  type Out

object EntityKey:
  type Aux[T <: Entity, Out0] = EntityKey[T] { type Out = Out0 }

  given [K]: EntityKey.Aux[Entity { type Key = K}, K] = null

// replacing the type with a trait
trait Dictionary[T <: Entity](using val entityKey: EntityKey[T]):
  type Dict = Map[entityKey.Out, T]

What does Dotty offer to replace type projections?
https://users.scala-lang.org/t/converting-code-using-simple-type-projections-to-dotty/6516
Dotty cannot infer result type of generic Scala function taking type parameter trait with abstract type