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Companion object of Serializable type becomes serializable too #9668

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scabug opened this issue Feb 22, 2016 · 3 comments
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Companion object of Serializable type becomes serializable too #9668

scabug opened this issue Feb 22, 2016 · 3 comments
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@scabug
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scabug commented Feb 22, 2016

File Test.scala:

package bug

package object testing {
  type Foo = String
  // alternatives:
  // trait Foo extends Serializable
  // type Foo <: String

  object Foo
}

object Test {
  def main(args: Array[String]): Unit = {
    // This should not display scala.Serializable
    println(testing.Foo.getClass.getInterfaces().mkString(", "))

    // This should not even compile:
    println(testing.Foo: Serializable)
  }
}

Compile and run:

doeraene@lamppc37:~/projects/temp/scalac-bug-serializable-companion$ ~/opt/scala-2.11.2/bin/scalac Test.scala
doeraene@lamppc37:~/projects/temp/scalac-bug-serializable-companion$ ~/opt/scala-2.11.2/bin/scala -cp . bug.Test
interface scala.Serializable

This can be particularly annoying in Scala.js, because it makes it impossible to declare a type alias whose companion is a JavaScript type (since JavaScript types cannot extend non-JS types such as Serializable). See scala-js/scala-js#2244 (comment)
@scabug
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scabug commented Feb 22, 2016

Imported From: https://issues.scala-lang.org/browse/SI-9668?orig=1
Reporter: @sjrd
Affected Versions: 2.11.2, 2.11.7

@scabug scabug added this to the Backlog milestone Apr 7, 2017
hrhino added a commit to hrhino/scala that referenced this issue Jan 1, 2020
@hrhino
Tests for scala#3899/scala#7686/scala#9668
8337f01 
I nearly resolved to clear the scala/bug backlog this year.

Fixes scala/bug#3899
Fixes scala/bug#7686
Fixes scala/bug#9668
@hrhino hrhino mentioned this issue Jan 1, 2020
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@som-snytt
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I see the linked PR was demonstrating that the example:

scalac t9668.scala && scala Test
t9668.scala:16: error: type mismatch;
 found   : testing.Foo.type
 required: Serializable
    println(testing.Foo: Serializable)
                    ^
one error found

because type aliases don't have companions.

But true companions are still made serializable.

@som-snytt
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scala/scala#5602 includes discussion confirming that the type alias fix was intended.

@SethTisue SethTisue removed this from the Backlog milestone Jan 28, 2020
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@SethTisue @som-snytt @scabug