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Possible Regression: Defining type covariant type alias fails when using in invariant position same compilation unit #10140

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scabug opened this issue Jan 10, 2017 · 6 comments
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Possible Regression: Defining type covariant type alias fails when using in invariant position same compilation unit #10140

scabug opened this issue Jan 10, 2017 · 6 comments
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@scabug
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scabug commented Jan 10, 2017 •
edited by dwijnand

This code used to work in Scala-2.11

type Id[+A] = A

trait Meh[F[_]] { def foo[A](a: => A): F[A] }

val idmeh = new Meh[Id] { 
    def foo[A](a: => A): Id[A] = a 
}

Now this fails with

<pastie>:18: error: covariant type Id occurs in invariant position in type => Meh[Id] of value idmeh
       val idmeh = new Meh[Id] {

However, If I change where I define type Id[+A] = A then it works.

scala> :paste
// Entering paste mode (ctrl-D to finish)

type Id[+A] = A

trait Meh[F[_]] { def foo[A](a: => A): F[A] }

// Exiting paste mode, now interpreting.

defined type alias Id
defined trait Meh

scala> val idmeh = new Meh[Id] {
     |     def foo[A](a: => A): Id[A] = a
     | }
idmeh: Meh[Id] = $anon$1@77fdbd9b

I also get this behaviour with real source files, not just in the repl.
@scabug
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scabug commented Jan 10, 2017

Imported From: https://issues.scala-lang.org/browse/SI-10140?orig=1
Reporter: @hamnis
Affected Versions: 2.12.1
See #8079

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scabug commented Feb 14, 2017

David Koeplinger (dkoeplin) said (edited on Feb 14, 2017 10:26:03 AM UTC):
Just ran into this as well. Another example:

trait X {
  type Id[+A] = A
  def foo[A](x: Id[A]) = x
}

error: covariant type Id occurs in contravariant position in type X.this.Id[A] of value x

However, if the type alias is defined in a separate trait, there's no error.

trait X { 
  type Id[+A] = A 
}
trait Y extends X {
  def foo[A](x: Id[A]) = x
}

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scabug commented Feb 15, 2017

@SethTisue said:
perhaps an effect of the fix for SI-8079?

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scabug commented Feb 15, 2017

@SethTisue said:
@retronym this might interest you

@scabug scabug added this to the 2.12.3 milestone Apr 7, 2017
@scabug scabug mentioned this issue Apr 7, 2017
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@retronym retronym modified the milestones: 2.12.3, 2.12.4 Jul 8, 2017
@benjaminfrank
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We have com about this recently as well, here is what we did:

// Does compile using Scala 2.11 (REPL)
// Does not compile using Scala 2.12 (REPL)
// Compiles using dotty (REPL)
trait TC[F[_]]; type F[+T] = T; new TC[F] {}

// Workaround for Scala 2.12 (REPL)
trait TC[F[_]]; object Foo {type F[+T] = T}; new TC[Foo.F] {}

@adriaanm adriaanm modified the milestones: 2.12.4, 2.12.5 Oct 10, 2017
@SethTisue SethTisue modified the milestones: 2.12.5, Backlog Mar 1, 2018
@dwijnand
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No longer the case

$ m test.scala
object Test {
  type Id[+A] = A

  trait Meh[F[_]] { def foo[A](a: => A): F[A] }

  val idmeh = new Meh[Id] {
      def foo[A](a: => A): Id[A] = a
  }
}
$ scalac -version
Scala compiler version 2.13.5 -- Copyright 2002-2020, LAMP/EPFL and Lightbend, Inc.
$ scalac test.scala
$

@dwijnand dwijnand removed this from the Backlog milestone Apr 30, 2021
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@retronym @adriaanm @SethTisue @dwijnand @benjaminfrank @scabug