build.sbt

scalaVersion := "2.11.8"

ClassFormatErrorBug.scala

package com.example

case class Foo[M[_], A](step: M[Foo.Step[A, Foo[M, A]]])

object Foo {
  class Step[A, S]
  object Done {
    def unapply[A, S](a: Step[A, S]): Boolean = true
  }
}

object ClassFormatErrorBug {
  implicit class AnyOps[A](actual: A) {
    def mustMatch(f: PartialFunction[A, Boolean]): Unit = {
    }
  }

  type Id[+A] = A

  def main(args: Array[String]): Unit = {
    val s = new Foo[Id, String](new Foo.Step[String, Foo[Id, String]])

    s.step mustMatch {
      case Foo.Done() => true
    }
  }
}