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Error on type projection C[_]#K #12463

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lrytz opened this issue Sep 16, 2021 · 6 comments
Open

Error on type projection C[_]#K #12463

lrytz opened this issue Sep 16, 2021 · 6 comments
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@lrytz
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fixed in Scala 3 This issue does not exist in the Scala 3 compiler (https://github.com/lampepfl/dotty/) infer
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@lrytz
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lrytz commented Sep 16, 2021 •
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import scala.language.higherKinds

trait B
class C[CC[_] <: B] {
  class K
}

object O {
  def t1(x: Any): Int = x match {
    case _: C[_]#K => 0
  }
  def t2(x: Any) = x.isInstanceOf[C[_]#K]
}

t1 fails in type checking, only if CC is higher-kinded

A.scala:10: error: type arguments [_] do not conform to class C's type parameter bounds [CC[_] <: B]
    case _: C[_]#K => 0
            ^

t2 fails in parsing

A.scala:12: error: unbound wildcard type
  def t2(x: Any) = x.isInstanceOf[C[_]#K]
                                    ^

Both work in Scala 3.

Interestingly, t1 compiles with Scala 2.12.14/15, but fails in the same way as 2.13.6 when adding the semanticdb-scalac compiler plugin (🤷‍♂️).

$> sc12 -Xplugin:/Users/luc/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/scalameta/semanticdb-scalac_2.12.15/4.4.28/semanticdb-scalac_2.12.15-4.4.28.jar A.scala
A.scala:10: error: type arguments [_] do not conform to class C's type parameter bounds [CC[_] <: B]
    case _: C[_]#K => 0
            ^
@lrytz lrytz added the infer label Sep 16, 2021
@lrytz lrytz added this to the 2.13.7 milestone Sep 16, 2021
@lrytz lrytz self-assigned this Sep 16, 2021
@som-snytt
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No diff with scala/scala#9712

@lrytz
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lrytz commented Sep 17, 2021 •
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Also in the same ballpark, works in Scala 3:

scala> class C[CC[_]]
scala> def f: C[_] = null
                ^
       error: _$1 takes no type parameters, expected: 1

@joroKr21
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Isn't that related to #8039?

@lrytz
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lrytz commented Sep 17, 2021

very much, thanks @joroKr21. It comes with one of @retronym's wip branches :-) https://github.com/retronym/scala/tree/ticket/8039

@lrytz lrytz mentioned this issue Sep 17, 2021
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@joroKr21
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@lrytz did you try that patch? I saw that you merged a workaround.

@lrytz
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lrytz commented Sep 27, 2021

@joroKr21 no, I didn't try the patch...

@SethTisue SethTisue modified the milestones: 2.13.7, 2.13.8 Oct 21, 2021
@dwijnand dwijnand added the fixed in Scala 3 This issue does not exist in the Scala 3 compiler (https://github.com/lampepfl/dotty/) label Nov 18, 2021
@SethTisue SethTisue modified the milestones: 2.13.8, 2.13.9 Dec 15, 2021
@SethTisue SethTisue modified the milestones: 2.13.9, 2.13.10, Backlog Apr 25, 2022
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@lrytz @SethTisue @dwijnand @som-snytt @joroKr21