gistfile1.scala

sealed trait Limited
sealed trait Unlimited
sealed trait Lim extends Limited with Unlimited

sealed trait Skipped
sealed trait Unskipped
sealed trait Sk extends Skipped with Unskipped

sealed trait Selected
sealed trait Unselected
sealed trait Sel extends Selected with Unselected

class AddLimit[-In, +Out]
implicit def addLimit[Rest >: Sel with Sk]: AddLimit[Rest with Unlimited, Rest with Limited] = null

class MaybeAddLimit[-In, +Out1, +Out2] extends AddLimit[In, Out1]
implicit def maybeAddLimit[Rest >: Sel with Sk]: MaybeAddLimit[Rest with Unlimited, Rest with Limited, Rest] = null

class AddSkip[-In, +Out]
implicit def addSkip[Rest >: Lim with Sel]: AddSkip[Rest with Unskipped, Rest with Skipped] = null

class AddSelect[-In, +Out]
implicit def addSelect[Rest >: Lim with Sk]: AddSelect[Rest with Unselected, Rest with Selected] = null

class Query[+T] {
  def limit[T2](n: Int)(implicit ev: AddLimit[T, T2]): Query[T2] = this.asInstanceOf[Query[T2]]
  def skip[T2](n: Int)(implicit ev: AddSkip[T, T2]): Query[T2] = this.asInstanceOf[Query[T2]]
  def select[T2]()(implicit ev: AddSelect[T, T2]): Query[T2] = this.asInstanceOf[Query[T2]]

  def fetch()(implicit ev: T <:< Limited): List[String] = Nil
}

val q = new Query[Unlimited with Unskipped with Unselected]

// I tried [State <: S3, S2 <: S3, S3] also.
// It inferred S3 to be Unlimited with Unskipped with Unselected and then couldn't find the implicit.
def maybeLimitQuery[State, S2, S3](b: Boolean, q: Query[State])
     (implicit ev: MaybeAddLimit[State, S2, S3]): Query[S3] = {
   val qq = if (b) 
     q.limit(3) // Query[S2]
   else 
     q          // Query[State]
 
   // State <: S3 and S2 <: S3, but the compiler doesn't know that
   qq.asInstanceOf[Query[S3]]
}

maybeLimitQuery(true, q)