Incorrect type error when reading Scala 2.13 library

[kubukoz]

Compiler version

3.0.0

Minimized code

sbt version: 1.5.2

//build.sbt
val library = project.settings(
  scalaVersion := "2.13.6"
)

val root = project
  .in(file("."))
  .settings(
    name := "demo",
    version := "0.1.0",
    scalaVersion := "3.0.0"
  )
  .dependsOn(library)

In the library module:

package cats.effect.kernel

trait Async[F[_]]

object Async {
  trait WriterTAsync[F[_], L1]
      extends Async[({ type LL[A] = WriterT[F, L1, A] })#LL]
      with MonadCancel.WriterTMonadCancel[F, L1] {

    override def delegate = super.delegate
  }

}

case class WriterT[F[_], L0, V]()

trait MonadError[F[_]]
trait MonadCancel[F[_]]

object MonadCancel {

  trait WriterTMonadCancel[F[_], L2]
      extends MonadCancel[({ type LL[A] = WriterT[F, L2, A] })#LL] {

    def delegate: MonadError[({ type LL[A] = WriterT[F, L2, A] })#LL] =
      ???

  }
}

In the root module:

import cats.effect.kernel.Async

object demo {

  def test1[F[_]](ev: Async[F]): Unit = ???
  def test2[F[_]](ev: Async[F]): Unit = ???

}

Output

sbt:demo> compile
[info] compiling 1 Scala source to /Users/jkozlowski/IdeaProjects/scala3-12641-reproduction/library/target/scala-2.13/classes ...
[info] compiling 1 Scala source to /Users/jkozlowski/IdeaProjects/scala3-12641-reproduction/target/scala-3.0.0/classes ...
[error] -- [E023] Syntax Error: /Users/jkozlowski/IdeaProjects/scala3-12641-reproduction/src/main/scala/demo.scala:6:22
[error] 6 |  def test2[F[_]](ev: Async[F]): Unit = ???
[error]   |                      ^^^^^^^^
[error]   |             Not enough type arguments for cats.effect.kernel.Async[F, L2]
[error]   |             expected: [F, L2]
[error]   |             actual:   [F]
[error] one error found
[error] one error found
[error] (Compile / compileIncremental) Compilation failed
[error] Total time: 17 s, completed 29 May 2021, 01:59:24

Expectation

Everything compiles fine.

Worth noting

• this doesn't happen when only one of test1 / test2 exists!

[kubukoz]

Also, is that really a syntax error? Feels more like a type error.

[odersky]

The error categorization was part of a community effort to improve error messages which means it's somewhat random. PRs appreciated to improve the situation. It would be good to try to minimize the problem further to pinpoint precisely where it fails. Gold standard is always a self-contained example.

[kubukoz]

I'll try :)

[kubukoz]

Updated with a zero-dependency reproduction in a single sbt build.