Force posNum and negNum generators to always return values

[zakpatterson]

Fixes (part of) #568.

[non]

This looks good. Would you be willing to add one other change? I think unless our Numeric[T] is actually a Fractional[T] we can do something even simpler, i.e.

def posNum[T](implicit num: Numeric[T], c: Choose[T]): Gen[T] = {
  import num._
  num match {
    case _: Fractional[_] =>
      sized(n => c.choose(zero, max(fromInt(n), one)).suchThat(_ != zero))
    case _ =>
      sized(n => c.choose(one, max(fromInt(n), one))
  }
}

The reason this might be better is that if n is one, the fractional types will still be able to generate plenty of non-zero values, but the integral types will be generating zero 50% of the time (resulting in more retries).

[zakpatterson]

Yes, that makes sense. I'm playing with it now and I'll push tonight.

[zakpatterson]

While I'm staring at this, is there a reason that negNum is not just

  def negNum[T](implicit num: Numeric[T], c: Choose[T]): Gen[T] = 
     posNum[T].map(num.negate(_))

[non]

@zakpatterson Using negation on negative numbers can be dangerous (since -Int.MinValue == Int.MinValue) but since this number will be positive I think your change makes sense.

This looks good. Thanks! 👍

[zakpatterson]

Thanks for the review @non, I learned something.

[non]

I think the build failure was transient, I'm retrying now.