Regression in Scala 2.13.7 - cannot use wildcard for F-bounded type when using OptManifest

[Sciss]

reproduction steps

using Scala 2.13.6 (and all before), the following works ( https://scastie.scala-lang.org/NkHsooECRGqIgttwtvkgMg )

trait Txn[T <: Txn[T]]

trait Universe[T <: Txn[T]]

object Ref {
  def apply[A](init: A)(implicit om: OptManifest[A]): Ref[A] = ???
}
trait Ref[A]

trait Handler {
  def all: Ref[Vector[Universe[_]]] = Ref(Vector.empty)   // here
}

In Scala 2.13.7, the marked line produces an error

type arguments [_$1] do not conform to trait Universe's type parameter bounds [T <: Txn[T]]
      def all: Ref[Vector[Universe[_]]] = Ref(Vector.empty)

This is somehow caused by the OptManifest argument to the Ref constructor. If I remove that implicit argument, it compiles again.

[Sciss]

Note that I can write in this case def all: Ref[Vector[Universe[_]]] = Ref(Vector.empty)(NoManifest) as a work-around, since the class manifest is not strictly needed here.

[som-snytt]

I'll take a look. Compare scalacenter/scalafix#1493

[som-snytt]

I guess it involves scala/scala#9776 and not wildcard representation. An extra check is introduced.

[SethTisue]

attn @lrytz

[som-snytt]

I'll let lrytz say if it is simply correct, which I guess it is. The manifest seeks to summon Class[Vector[Universe[_]]]. I've misplaced my debug output. The test shows the correctly conforming case.

test at scala/scala#9802

[lrytz]

Thank you @som-snytt - I think you're right

  def m1[T <: Txn[T]] = optManifest[Universe[T]] // ok
  def m2 = optManifest[Universe[_]] // type arguments [_$2] do not conform to trait Universe's type parameter bounds [T <: Txn[T]]

[lrytz]

Though i'm not all that sure how wildcards are working / supposed to work. def t: Universe[_] = ??? is ok. Writing out the code after -Vprint:typer explicitly type checks.

def t: Ref[Vector[Universe[_]]] = Ref.apply[Vector[Universe[_]]](scala.Vector.empty[Nothing])(scala.reflect.ClassManifestFactory.classType[Vector[Universe[_]]](classOf[scala.collection.immutable.Vector[_]], scala.reflect.ClassManifestFactory.classType[Universe[_]](classOf[Universe[_]], scala.reflect.NoManifest)))

[Jasper-M]

Works in Scala 3, by the way. But their wildcards are generally better at ignoring stuff, as demonstrated in the possibly somewhat related #8039.

[som-snytt]

Yes, I noticed that dotty is nicer with special wildcard name handling. I expected to need some name-based extraction somewhere. But per Lukas's comment, I tried to induce explicit failure, but it was something else leaking when summoning the manifest, which is not merely implicit but "internal" or "magical". Also, paulp's "rocket science" dig is motivational, I don't remember why it didn't come up on the underscore ticket.

BTW both PRs landed about the same time, which is why I assumed the underscore ticket was causative. I didn't actually bisect.

[joroKr21]

The main code path where checkTypeRef is used takes care of converting existential types to wildcards:
https://github.com/scala/scala/blob/2.13.x/src/compiler/scala/tools/nsc/typechecker/RefChecks.scala#L1758-L1780

            val existentialParams = new ListBuffer[Symbol]
            var skipBounds = false
            // check all bounds, except those that are existential type parameters
            // or those within typed annotated with @uncheckedBounds
            if (!inPattern) tree.tpe foreach {
              case tp @ ExistentialType(tparams, tpe) =>
                existentialParams ++= tparams
              case ann: AnnotatedType if ann.hasAnnotation(UncheckedBoundsClass) =>
                // scala/bug#7694 Allow code synthesizers to disable checking of bounds for TypeTrees based on inferred LUBs
                // which might not conform to the constraints.
                skipBounds = true
              case tp: TypeRef =>
                val tpWithWildcards = deriveTypeWithWildcards(existentialParams.toList)(tp)
                checkTypeRef(tpWithWildcards, tree, skipBounds)
              case _ =>
            }
            if (skipBounds) {
              tree.setType(tree.tpe.map {
                _.filterAnnotations(_.symbol != UncheckedBoundsClass)
              })
            }

            tree

[joroKr21]

TIL that this doesn't complain 🤔

  trait Foo[A <: Int]
  class bar[A] extends StaticAnnotation
  @bar[Foo[String]]
  val x = 42

[joroKr21]

Also this:

  trait Foo[A <: Int]
  trait Bar[A, B]
  val x: Bar[Foo[String] @uncheckedBounds, Foo[String]] = null

[SethTisue]

(leaving it on the 2.13.8 milestone, because no released version of 2.12 had the bug)