different behavior between typelevel and standard compiler

typelevel-difference.scala

/** The following difference in behavior is seen between the
 *  typelevel scala 2.12.1 compiler and the standard one. The
 *  only difference between the failing case and the succeeding one
 *  is the scalaOrganization. No compiler options are needed.
 *  To reproduce, add
 *
 *    scalaOrganization := "org.typelevel" 
 *
 *  to the build.sbt at https://github.com/paulp/psp-std and sbt test.
 */

// This test case fails because typelevel chooses a less specific Show instance.
showsAs("[1..3]", 1 nthTo 3)

// Trying to pass the show instance which the standard compiler chooses shows why.
showsAs("[1..3]", 1 nthTo 3)(showConsecutive)

[error] /g/psp-std/std/src/test/scala/tests/MiscSpec.scala:81: type mismatch;
[error]  found   : psp.std.Show[psp.std.Consecutive[_ <: Singleton]]
[error]  required: psp.std.Show[psp.std.all.ClosedRange[psp.std.Nth]]
[error]     (which expands to)  psp.std.Show[psp.std.Consecutive.Closed[psp.std.Nth]]
[error]     showsAs("[1..3]", 1 nthTo 3)(showConsecutive),
[error]                                  ^

// However if I pass type arguments to showConsecutive, typelevel
// is ok and the test passes.
showsAs("[1..3]", 1 nthTo 3)(showConsecutive[Nth, ClosedRange])

// Here's the definition of showConsecutive, complete with ominous comment.

/** Simpler versions of this method signature lead to implicit ambiguity.
 */
implicit def showConsecutive[A, CC[X] <: Consecutive[X]] : Show[CC[_ <: A]] = by(_.in)

/**** So... Where's the Singleton bound coming from? ****/