Flakey implicit resolution with type alias Id[A] = A

[scabug]

In the code below, implicit search for Monad[Id] succeeds by itself, but not always when it is part of a recursive search.

Most of the time it is impractical to define a type alias and annotate values with this type alias.

object Test {

  // Data types

  type Id[A] = A

  class MaybeT[F[_], A]

  type Maybe[A] = MaybeT[Id, A]

  type MaybeMaybe[A] = MaybeT[Maybe, A]


  // Typeclass

  trait Monad[F[_]]


  // Instances

  implicit val monadId: Monad[Id] = ???

  implicit def monadMaybeT[F[_]: Monad]: Monad[({ type λ[A] = MaybeT[F, A] })#λ] = ???

  implicit val monadOption: Monad[Option] = ???


  // Implicit search tests

  // OK
  implicitly[Monad[Id]]
  implicitly[Monad[({ type λ[A] = A })#λ]]

  // OK
  implicitly[Monad[Maybe]]

  // ERROR, despite asking for the same thing as before, but using a type lambda
  // error: could not find implicit value for parameter e: Test.Monad[[A]Test.MaybeT[[A]A,A]]
  implicitly[Monad[({ type λ[A] = MaybeT[Id, A] })#λ]]
  // implicitly[Monad[MaybeT[Id, ?]]]

  // OK
  implicitly[Monad[MaybeMaybe]]

  // ERROR, despite asking for the same thing as before, but using a type lambda
  // error: could not find implicit value for parameter e: Test.Monad[[A]Test.MaybeT[[A]Test.MaybeT[[A]A,A],A]]
  implicitly[Monad[({ type λ[A] = MaybeT[Maybe, A] })#λ]]
  // implicitly[Monad[MaybeT[Maybe, ?]]]

  // OK
  // Option instead of Maybe works,
  // because Option isn't a type alias containing Id.
  implicitly[Monad[({ type λ[A] = MaybeT[Option, A] })#λ]]
  // implicitly[Monad[MaybeT[Option, ?]]]
}

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-10238?orig=1
Reporter: Tomas Mikula (tomas.mikula-at-gmail.com)
Affected Versions: 2.12.1