java.lang.ClassFormatError: Duplicate method name&signature when upgrading to cats 1.0.0-MF

[julien-truffaut]

Hi all,

When @sellout tried to upgrade newts (https://github.com/julien-truffaut/newts) from cats 0.9.0 to 1.0.0-MF, we started to experience java.lang.ClassFormatError when calling show on case class using AnyVal e.g.:

final case class First[A](getFirst: A) extends AnyVal
object First {
  implicit def showInstance[A](implicit ev: Show[A]): Show[First[A]] = new Show[First[A]] {
    override def show(f: First[A]): String = s"First(${ev.show(f.getFirst)})"
  }
}

First("aString").show

here is the exact error:

[info] newts.FirstTest *** ABORTED ***
[info]   java.lang.ClassFormatError: Duplicate method name&signature in class file newts/First$$anon$1
[info]   at java.lang.ClassLoader.defineClass1(Native Method)
[info]   at java.lang.ClassLoader.defineClass(ClassLoader.java:760)

Now what's interesting is that the error disappear if you:

• remove Anyval or
• change uses Show.show e.g.

implicit def showInstance[A](implicit ev: Show[A]): Show[First[A]] =  Show.show(a =>
    s"First(${Show[A].show(a.getFirst)})"
  )

For precision, everything worked fine in 0.9.0

[kailuowang]

Confirmed it's not simulacrum. A possible work around at the cats side is to rename the show in ContravariantShow. In the meantime, @julienrf @sellout can you guys use the Show.show work around before next release?

[julien-truffaut]

sure PR is already pending. I was mostly creating this issue in case in anyone else face it.

[dwijnand]

Confirmed it's not simulacrum. A possible work around at the cats side is to rename the show in ContravariantShow.

An alternative workaround is to make simulacrum.typeclass see the original show in ContravariantShow.