-Xsource:3 on scala 2.13.6 breaks type inference for e.g. scala.js union

[nafg]

This succeeds without the flag and fails with it:

  class OptGroup[A]
  type Opt[A] = A | OptGroup[A]

  def setting(v: Boolean | Seq[Opt[Int]]) = ()
  def v: Seq[Opt[Int]] = ???
  setting(v)

This takes about a second to compile (successfully) without it, and about an hour with it (I forgot if it succeeds or fails):

  // Minimized code from Scalably Typed bindings for Ant Design
  trait RenderedCell[RecordType]
  object ColumnType {
    def apply[RecordType](): ColumnType[RecordType] = ???

    implicit class ColumnTypeMutableBuilder[Self <: ColumnType[_], RecordType](
        val x: Self with ColumnType[RecordType]
    ) extends AnyVal {
      def setRender(
          value: (Any, RecordType, Double) => Node | RenderedCell[RecordType]
      ): Self = ???

    }
  }
  trait ColumnType[RecordType]

  // Minimized application code
  def render: Double => VdomNode = ???
  ColumnType[Double]()
    .setRender((_, t, _) => render(t).rawNode
    // .asInstanceOf[Node | RenderedCell[Double]]
    )

A standalone reproduction that does not require scalajs or any library is here: https://gitlab.com/nafg/repro-xsource3-inference

[nafg]

You can see the behavior at https://gitlab.com/nafg/repro-xsource3-inference/-/pipelines/346233753/builds

[dwijnand]

Fails with:

t12437.scala:7: error: type mismatch;
 found   : Seq[Test.Opt[Int]]
    (which expands to)  Seq[Int | Test.OptGroup[Int]]
 required: Boolean | Seq[Test.Opt[Int]]
    (which expands to)  Boolean | Seq[Int | Test.OptGroup[Int]]
  setting(v)
          ^
1 error

[lrytz]

This is due to scala/scala#6037

https://github.com/scala/scala/blob/3081265e666707fe3f6467200ed46afa8a9dbfa0/src/compiler/scala/tools/nsc/typechecker/Infer.scala#L907-L929

[smarter]

Minimized, with | replaced by ^ to avoid confusion with scala 3's |:

import scala.language.implicitConversions

sealed trait ^[A, B] // scalastyle:ignore

object ^ { // scalastyle:ignore
  /** Evidence that `A <: B`, taking `^`-types into account. */
  sealed trait Evidence[-A, +B]

  /** A unique (and typically dead-code-eliminated away) instance of
   *  `Evidence`.
   */
  private object ReusableEvidence extends Evidence[scala.Any, scala.Any]

  abstract sealed class EvidenceLowestPrioImplicits { this: Evidence.type =>
    /** If `A <: B2`, then `A <: B1 ^ B2`. */
    implicit def right[A, B1, B2](implicit ev: Evidence[A, B2]): Evidence[A, B1 ^ B2] =
      ReusableEvidence.asInstanceOf[Evidence[A, B1 ^ B2]]

    /** Given a covariant type constructor `F[+_]`, if `A <: B`, then
     *  `F[A] <: F[B]`.
     */
    implicit def covariant[F[+_], A, B](implicit ev: Evidence[A, B]): Evidence[F[A], F[B]] =
      ReusableEvidence.asInstanceOf[Evidence[F[A], F[B]]]

    // /** Given a contravariant type constructor `F[-_]`, if `B <: A`, then
    //  *  `F[A] <: F[B]`.
    //  */
    // implicit def contravariant[F[-_], A, B](implicit ev: Evidence[B, A]): Evidence[F[A], F[B]] =
    //   ReusableEvidence.asInstanceOf[Evidence[F[A], F[B]]]
  }

  abstract sealed class EvidenceLowPrioImplicits
      extends EvidenceLowestPrioImplicits {
    this: Evidence.type =>

    // /** `Int <: Double`, because that's true in Scala.js. */
    // implicit def intDouble: Evidence[Int, Double] =
    //   ReusableEvidence.asInstanceOf[Evidence[Int, Double]]

    // /** If `A <: B1`, then `A <: B1 ^ B2`. */
    // implicit def left[A, B1, B2](implicit ev: Evidence[A, B1]): Evidence[A, B1 ^ B2] =
    //   ReusableEvidence.asInstanceOf[Evidence[A, B1 ^ B2]]
  }

  object Evidence extends EvidenceLowPrioImplicits {
    /** `A <: A`. */
    implicit def base[A]: Evidence[A, A] =
      ReusableEvidence.asInstanceOf[Evidence[A, A]]

    // /** If `A1 <: B` and `A2 <: B`, then `A1 ^ A2 <: B`. */
    // implicit def allSubtypes[A1, A2, B](
    //     implicit ev1: Evidence[A1, B], ev2: Evidence[A2, B]): Evidence[A1 ^ A2, B] =
    //   ReusableEvidence.asInstanceOf[Evidence[A1 ^ A2, B]]
  }

  /** Upcast `A` to `B1 ^ B2`.
   *
   *  This needs evidence that `A <: B1 ^ B2`.
   */
  implicit def from[A, B1, B2](a: A)(implicit ev: Evidence[A, B1 ^ B2]): B1 ^ B2 =
    a.asInstanceOf[B1 ^ B2]

  // /** Upcast `F[A]` to `F[B]`.
  //  *
  //  *  This needs evidence that `F[A] <: F[B]`.
  //  */
  // implicit def fromTypeConstructor[F[_], A, B](a: F[A])(
  //     implicit ev: Evidence[F[A], F[B]]): F[B] =
  //   a.asInstanceOf[F[B]]

  // /** Operations on union types. */
  // implicit class UnionOps[A <: _ ^ _] private[^] (private val self: A)
  //     extends AnyVal {

  //   /** Explicitly merge a union type to a supertype (which might not be a
  //    *  union type itself).
  //    *
  //    *  This needs evidence that `A <: B`.
  //    */
  //   def merge[B](implicit ev: ^.Evidence[A, B]): B =
  //     self.asInstanceOf[B]
  // }
}

class OptGroup[A]
object Test {
  type Opt[A] = A ^ OptGroup[A]

  def setting(v: Boolean ^ Seq[Opt[Int]]) = ()
  def v: Seq[Opt[Int]] = ???
  setting(v)
  // synthesized: setting(^.from[Seq[Test.Opt[Int]], Boolean, Seq[Test.Opt[Int]]](Test.this.v)(^.this.Evidence.right[Seq[Test.Opt[Int]], Boolean, Seq[Test.Opt[Int]]](^.this.Evidence.base[Seq[Test.Opt[Int]]])))
}

The problem goes away if implicit def covariant is commented out, the problem is also unique to Scala 2's -Xsource:3, this code compiles fine in Scala 3. So it seems like scala/scala#6037 does not fully match the semantics of Scala 3, but I don't know why /cc @milessabin @sjrd .

[smarter]

If no one can figure out how to fix this, then I suggest removing or hiding behind a Y flag the implicit resolution change currently in Xsource:3

[dwijnand]

@nafg I assume this is impacting you in one of your projects. Can I interest you in submitting a PR making the suggested change?

[povder]

Is there a good chance of a fix for this landing in 2.13.9? I considered contributing by implementing #12437 (comment) but after reading through the code I don't think I am capable of doing that. I'd guess that most people are hitting this when using Scala.js union so maybe a workaround is possible to implement on scala-js/scala-js side? @sjrd

[povder]

I considered contributing by implementing #12437 (comment) but after reading through the code I don't think I am capable of doing that.

Actually, I'll give it a shot, I guess I don't need to understand the actual content of scala/scala#6037 to hide it behind an -Y flag.