Phase consistency involving higher-kinded types

[liufengyun]

When a higher-kinded type S is used in quote/splice, do we need an evidence of Type[S] or Type[S[X]]? Neither type checks.

import scala.quoted._
import scala.tasty._

class MatchFactory1[T, S[_]]

object MatcherFactory1 {
  def impl[T: Type, S[_]](self: Expr[MatchFactory1[T, S]])(implicit refl: Reflection, tpS: Type[S[T]]) =
    '{ ~self }
}

[liufengyun]

Related: phase consistency for path-dependent types #5961 (comment)

[nicolasstucki]

You can encode the missing type tag with M >: MatchFactory1[T, S] <: MatchFactory1[T, S] : Type

import scala.quoted._
import scala.tasty._

class MatchFactory1[T, S[_]] {
  def f: Int = 2
}

object MatcherFactory1 {

  def impl[T: Type, S[_], M >: MatchFactory1[T, S] <: MatchFactory1[T, S] : Type](self: Expr[M])(implicit refl: Reflection, tpS: Type[S[T]]) =
    '{ val a = ~self; a.f }

}

[liufengyun]

@nicolasstucki The encoding does not compile when there is inner class, type projection only allowed on non-concrete types under Scala2 mode:

import scala.quoted._
import scala.tasty._

class MatchFactory1[T, S[_]] { outer =>
  class Inner {
    val owner = outer
  }
}

object MatcherFactory1 {

  def impl[T: Type, S[_], M >: MatchFactory1[T, S] <: MatchFactory1[T, S] : Type](self: Expr[M#Inner])(implicit refl: Reflection, tpS: Type[S[T]]): Expr[M] =
    '{ (~self).owner }

}

We could move #Inner to the bounds as well, but it becomes a little verbose.

[liufengyun]

@odersky Mentioned we could change the parameter of Type to poly-kinded:

sealed abstract class Type[T <: AnyKind] {
  type unary_~ = T
}

class Test {
  def foo(ev: Type[List]): Int = ???
}

However, this requires enable the flag -Ykind-polymorphism.