Inference limitation, distilled from https://groups.google.com/d/msg/scala-user/WFkqE1KpnGA/TC9tvLvXNcQJ

test.scala

trait Test {
  type Mapped[F[_]]
  type sx[T] = T => String
  trait fx[X] {
    type λ[T] = T => X
  }
  def summon[F[_]]: Mapped[F]

  // okay. Inference can unify the higher kinded type variable ?F with sx
  summon : Mapped[sx]           

  // fails
  summon : Mapped[fx[String]#λ]

  // Why?
  //
  // `Mapped[fx[String]#λ]` is typechecked to `Test.this.Mapped[[T]T => String]`
  //
  // This happens in `typedSelect` -> `checkAccessible`, in which this computation occurs
  //
  // ------------------------------------------------------------------------------------
  // scala> type Mapped[F[_]]
  //  type sx[T] = T => String
  // defined type alias Mapped
  // defined type alias sx
  //
  // scala>   trait fx[X] {
  //      |     type λ[T] = T => X
  //      |   }
  // defined trait fx
  //
  // scala> :power
  // ** Power User mode enabled - BEEP WHIR GYVE **
  // ** :phase has been set to 'typer'.          **
  // ** scala.tools.nsc._ has been imported      **
  // ** global._, definitions._ also imported    **
  // ** Try  :help, :vals, power.<tab>           **
  //
  // scala> val pre = typeOf[fx[String]]
  // pre: $r.intp.global.Type = fx[String]
  //
  // scala> val sym = pre.member(TypeName("λ").encodedName)
  // sym: $r.intp.global.Symbol = type λ
  //
  // scala> pre memberType sym
  // res0: $r.intp.global.Type = [T]T => String
  //
  // scala> res0.getClass
  // res1: Class[_ <: $r.intp.global.Type] = class scala.reflect.internal.Types$PolyType
  //
  // scala> showRaw(res0)
  // res2: String = PolyType(List(TypeName("T")), TypeRef(ThisType(scala), scala.Function1, List(TypeRef(NoPrefix, TypeName("T"), List()), TypeRef(ThisType(java.lang), java.lang.String, List()))))
  // ------------------------------------------------------------------------------------
  //
  // The compiler would need to infer `F= [T]T => String`, but it never infers a polytype
  // as a type argument as we don't have higher order unification. (https://issues.scala-lang.org/browse/SI-2712)
  // 
}