Simple check leads to infinite loop

[danielkarch]

Hi,

I just started playing around with ScalaCheck and encountered a problem. The following does not terminate on my machine with ScalaCheck 1.13.[0|1]:

import org.scalacheck.Properties
import org.scalacheck.Prop.forAll

object HelloSpec extends Properties("Hello") {

  def replaceLettersWithX(str: String) = {

    def replaceLetter(ch: Char): Char = {
      if (ch.isLetter) {
        'X'
      } else {
        ch
      }
    }

    str map replaceLetter
  }

  property("is actually the identity function") = forAll { (str: String) =>
    val withX: String = replaceLettersWithX(str)
    withX == str
  }

}

With version 1.12.5, a counter-example is produced quickly. With the newer versions, it gets stuck in an infinite loop. I tried to debug it and it seems like once ScalaCheck has found a counter-example, it is trying to shrink it. The shrinking however only keeps replacing one 1-character string with another.