dotc makes every single object extend java.io.Serializable, which it shouldn't.

[sjrd]

Minimized code

object Test

Output

sbt:dotty> dotc -Xprint:typer tests/run/hello.scala
[warn] Multiple main classes detected.  Run 'show discoveredMainClasses' to see the list
[info] running (fork) dotty.tools.dotc.Main -classpath C:\Users\sjrdo\AppData\Local\Coursier\cache\v1\https\repo1.maven.org\maven2\org\scala-lang\scala-library\2.13.3\scala-library-2.13.3.jar;C:\Users\sjrdo\Documents\Projets\dotty\library\..\out\bootstrap\dotty-library-bootstrapped\scala-0.27\dotty-library_0.27-0.27.0-bin-SNAPSHOT.jar -Xprint:typer tests/run/hello.scala
result of tests\run\hello.scala after typer:
package <empty> {
  final lazy module val Test: Test$ = new Test$()
  final module class Test$() extends Object(), _root_.scala.Serializable {
    this: Test.type =>
  }
}
[success] Total time: 4 s, completed 19 ao¹t 2020 18:11:54

$ javap -cp . Test$
Compiled from "hello.scala"
public final class Test$ implements java.io.Serializable {
  public static final Test$ MODULE$;
  public static {};
}

Expectation

I expect Test not to extend java.io.Serializable. It was not asked for in the source code.

scalac does not make objects java.io.Serializable like that.

This is very problematic for Scala.js because JS objects cannot extend java.io.Serializable, which is a Scala class.

[smarter]

#9600 fixed the scala.js issue, closing since I think having objects extend Serializable in the JVM backend is legitimate (cf the comment added in the PR)