I didn't see this. You can copy the unit test. It passed locally.

https://github.com/scala/scala/compare/2.12.x...som-snytt:issue/10086-max-order?expand=1

It's not obvious to me that the following min should be slow-path. If the ordering passed to min/max is the constructor Integral, ord eq num, can it just return start/last. (I noticed Numeric was not always an Ordering.)

scala> import scala.collection.immutable.NumericRange, math._
import scala.collection.immutable.NumericRange
import math._

scala> val sz = 1000000000
sz: Int = 1000000000

scala> implicit val customIntegral = new Numeric.IntIsIntegral with Ordering.IntOrdering { }
customIntegral: scala.math.Numeric.IntIsIntegral with scala.math.Ordering.IntOrdering = $anon$1@127a7272

scala> val nr = NumericRange(1, sz, 1)
nr: collection.immutable.NumericRange.Exclusive[Int] = NumericRange 1 until 1000000000

scala> nr.min
res0: Int = 1

This is ready for review.

Probably review by @Ichoran "Ichorange".

It would be good to have something--at least a commit message--that explains why (ord eq num) could be true. It's not utterly obvious.

Updated. Please let me know when you would like me to squash the commits.

LGTM, squashing on merge.