Summoning Mirror for ADT defined in trait

[TimWSpence]

Compiler version

scala-3.1.0-RC1-bin-20210817-f27c250-NIGHTLY (I tried to use the most recent I could find as I thought there's a chance that #11686 might have fixed this as it's in a similar area. Not entirely sure if that made it into this nightly or not)

Minimized code

import scala.deriving.Mirror

class Scope extends IListDefn {

  type Of = Mirror { type MirroredType[X] = IList[X]; type MirroredMonoType = IList[Any] ; type MirroredElemTypes[_] <: Tuple }

  val M = summon[Of]
}


trait IListDefn {
  sealed trait IList[A]
  case class INil[A]() extends IList[A]
  case class ICons[A](h: A, t: IList[A]) extends IList[A]
}

Output

[error] -- Error: /private/tmp/scala/Minimization.scala:7:20 ---------------------------
[error] 7 |  val M = summon[Of]
[error]   |                    ^
[error]   |no implicit argument of type Scope.this.Of was found for parameter x of method summon in object Predef
[error] one error found

Expectation

Compilation should succeed (and does if you move the IList definition out of the trait and up to top-level)

[bishabosha]

workaround is to give a companion object to IList (which acts as the Mirror object that will be summoned). Otherwise e.g. the child INil fails the accessibility test for generating an anonymous class Mirror type at the callsite M, perhaps that test is too restrictive in some cases

[TimWSpence]

Thanks @bishabosha! Unfortunately this is ultimately for https://github.com/typelevel/kittens where we summon Mirror instances via shapeless 3 for typeclass derivation so we have no control over how users define their ADTs.

[milessabin]

@bishabosha is there any chance we could tweak the accessibility test before 3.1?

[bishabosha]

if we tweak the accessibility rules to allow this case, then changing IList to be monomorphic will crash the compiler:

class Scope2 extends ZListDefn {

  type Of = Mirror { type MirroredType = ZList; type MirroredMonoType = ZList ; type MirroredElemTypes <: Tuple }

  val M = summon[Of]

}

class ZListDefn {
  sealed trait ZList
  case class ZNil() extends ZList
  case class ZCons(h: Boolean, t: ZList) extends ZList
}

with the same cause as #12328, summarised by my comment here #12328 (comment), the fix is more involved as it requires substituting the correct prefix into the types of the sealed children

[odersky]

Yes, for the moment this is simply out of spec. We generate mirrors only for statically accessible classes. I would advise to go slowly here and make absolutely sure that each generalization step can be made without negative consequences.