f-bounds and wildcards don't mix?

[adriaanm]

I think the following should compile (it does in 2.13), but it produces the following errors (note the LazyRef in the error message):

  trait BS[T, S <: BS[T, S]]
  trait IS extends BS[Int, IS]

  sealed trait BSElem[T, S <: BS[_, S]] // we cannot make this S <: BS[T, S], as the `_` could be something else from T (that's what the `BSElem` implicit is for)

  /*
-- [E057] Type Mismatch Error: /Users/adriaan/git/scala/src/library/scala/collection/convert/StreamExtensions.scala:34:36 
34 |  sealed trait BSElem[T, S <: BS[_, S]]
   |                                    ^
   |Type argument S does not conform to upper bound StreamExtensions.this.BS[Any, LazyRef(S)] 
*/

  object BSElem {
    implicit val intStreamShape: BSElem[Int, IS] = ???
  }
  class Ops[A] {
    def asJavaSeqStream[S <: BS[_, S]](implicit s: BSElem[A, S]): S = ???
    /*
  -- [E057] Type Mismatch Error: /Users/adriaan/git/scala/src/library/scala/collection/convert/StreamExtensions.scala:39:35 
  39 |    def asJavaSeqStream[S <: BS[_, S]](implicit s: BSElem[A, S]): S = ???
     |                                   ^
     |Type argument S does not conform to upper bound StreamExtensions.this.BS[Any, LazyRef(S)] 
    */
  }

[adriaanm]

This is blocking scala/scala#7458

[smarter]

@adriaanm @lrytz Any reason why the following wouldn't work:

trait BS[T, S <: BS[T, S]]
trait IS extends BS[Int, IS]

sealed trait BSElem[T, S <: BS[T, S]]

object BSElem {
  implicit val intStreamShape: BSElem[Int, IS] = ???
}
class Ops[A] {
  def asJavaSeqStream[S <: BS[A, S]](implicit s: BSElem[A, S]): S = ???
}

[adriaanm]

Sadly, no — I tried that. It breaks down when the implicit is used for computing element types of streams with wider numeric types (byte stream for example)

…

On Fri, 22 Mar 2019 at 17:37, Guillaume Martres ***@***.***> wrote: @adriaanm <https://github.com/adriaanm> @lrytz <https://github.com/lrytz> Any reason why the following wouldn't work: trait BS[T, S <: BS[T, S]]trait IS extends BS[Int, IS] sealed trait BSElem[T, S <: BS[T, S]] object BSElem { implicit val intStreamShape: BSElem[Int, IS] = ??? }class Ops[A] { def asJavaSeqStream[S <: BS[A, S]](implicit s: BSElem[A, S]): S = ??? } — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#6146 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAFjyyMYHPI9yVHGdnyqw7wYxec_hu8Sks5vZQbigaJpZM4cDo74> .

[lrytz]

Yes, I tried that as well...

[smarter]

A full example would be helpful

[lrytz]

Here's a full example that, I think, reflects the situation. The source of the problem might be because of the use of Integer (BS/AS/IS are defined in Java in the original). The version below compiles with scalac, but not dotty. Use the two commented-out lines instead is what you suggest above, and then the implicit is not found. Maybe it's just a matter of adding more implicit instances (for Integer), I'll check later - don't have time right now anymore :-)

trait St[A]
trait IntSt extends St[Int]
trait AnySt[T] extends St[T]

trait StS[T, S <: St[_]]
trait StSL {
  implicit def anyStS[T]: StS[T, AnySt[T]] = ???
}
object StS extends StSL {
  implicit val intStS: StS[Int, IntSt] = ???
}

trait BS[T, S <: BS[T, S]]
trait AS[T] extends BS[T, AS[T]]
trait IS extends BS[java.lang.Integer, IS]

trait SS[T, S <: BS[_, S], Z <: St[_]]
// trait SS[T, S <: BS[T, S], Z <: St[_]]
trait SSL {
  implicit def anySS[T]: SS[T, AS[T], AnySt[T]] = ???
}
object SS extends SSL {
  implicit val iss: SS[Int, IS, IntSt] = ???
}

object T {
  implicit class ListExt[A](l: List[A]) {
    def ext[S <: BS[_, S], Z <: St[_]](implicit sh: SS[A, S, Z], sth: StS[A, Z]): S = ???
    // def ext[S <: BS[A, S], Z <: St[_]](implicit sh: SS[A, S, Z], sth: StS[A, Z]): S = ???
  }
}

object U {
  import T._
  List(1).ext: IS
}

[lrytz]

Fixed in the PR by relaxing the bound

trait SS[T, S <: BS[_, _], Z <: St[_]]

...

    def ext[S <: BS[_, _], Z <: St[_]](implicit sh: SS[A, S, Z], sth: StS[A, Z]): S = ???

I cannot answer if dotty is right to report an error in the original example in the issue description.