ClassCastException (ArraySeq) when pattern-matching on varargs case class

[cheeseng]

We encounter a weird ClassCastException when running our tests in ScalaTest for org.scalactic.Every, that looks like this:

[error] (run-main-0) java.lang.ClassCastException: scala.collection.immutable.ArraySeq$ofInt cannot be cast to com.test.Every
[error] java.lang.ClassCastException: scala.collection.immutable.ArraySeq$ofInt cannot be cast to com.test.Every
[error] 	at com.test.TestMain$.main(TestMain.scala:7)
[error] 	at com.test.TestMain.main(TestMain.scala)
[error] 	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
[error] 	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
[error] 	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
[error] 	at java.lang.reflect.Method.invoke(Method.java:498)
[error] Nonzero exit code: 1
[error] (Compile / run) Nonzero exit code: 1

I tried to minimize the problem but not quite successful, the best I can do is to include the source of Every as com.test.Every, in the attached zip (a sbt project), you can reproduce the problem by running:

> sbt run

If you change the scala version to 2.12.8, it works without problem.

test-bug.zip

[Jasper-M]

This is ... interesting.

scala> case class Foo(a: Int*) { def lengthCompare(i: Int) = 1 }
defined class Foo

scala> Foo(1) match { case Foo(a) => a }
java.lang.ClassCastException: scala.collection.immutable.ArraySeq$ofInt cannot be cast to Foo
  ... 36 elided

[eed3si9n]

@Jasper-M Nice minimization.

[SethTisue]

culprit guess, is it scala/scala#7068? (refs #11040)

note that this isn't an RC1 regression; it was broken in M5 too

@adriaanm suspects https://github.com/scala/scala/pull/7068/files#diff-352de7f9c77a7e21df6940d6a1bcdc7eR574

@eed3si9n wonders if scala/scala#6485 is also related

[SethTisue]

@cheeseng is this hard to work around in your codebase? wondering how much of a 2.13 blocker it is

[adriaanm]

Well, “suspect” is too strong right now. Maybe the PR was just in the wrong place at the wrong time :-)

…

On Mon, 8 Apr 2019 at 16:08, Seth Tisue ***@***.***> wrote: @cheeseng <https://github.com/cheeseng> is this hard to work around in your codebase? wondering how much of a 2.13 blocker it is — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#11476 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAFjy1CE4q9zT5fmV3q-MZm6WpqrFcwwks5ve2lsgaJpZM4ciV4j> .

[cheeseng]

@SethTisue Not really a blocker, we just want to understand the reason behind tests failing under 2.13, and this is one of them.