Unexpected diverging implicit with singleton type

[alexarchambault]

Reported by @raulraja on the gitter of shapeless (https://gitter.im/milessabin/shapeless?at=56fcf50fbbffcc665faad6e5)

import org.scalacheck.Shapeless._
import shapeless._
import org.scalacheck._

def test[L <: HList : Arbitrary](l : L) : Arbitrary[L] = implicitly[Arbitrary[L]]

test(1 :: HNil) // works

import shapeless.syntax.singleton._

test(1.narrow :: HNil)

Surprisingly, explicitly specifying the type parameter of test makes this work again,

test[Witness.`1`.T :: HNil](1.narrow :: HNil) // fine

[alexarchambault]

@milessabin Does this ring a bell to you? This looks quite similar to #7 (comment), although removing arbContainer2 from scalacheck-shapeless doesn't address the issue here (one just gets "cannot find implicit value..." then).

Unlike #7, the problem only appears when singleton types are involved.

[milessabin]

No, I've not seen this before. It might be my fault, but I'm tempted to call this a compiler bug. Here's something else which works,

scala> type One = Witness.`1`.T
defined type alias One

scala> test((1: One) :: HNil)
res7: org.scalacheck.Arbitrary[shapeless.::[One,shapeless.HNil]] = org.scalacheck.ArbitraryLowPriority$$anon$1@595812b9

Also note,

scala> 1.narrow
res8: Int(1) = 1

scala> (1.narrow: One)
res9: One = 1

And again,

scala> def typed[T](t: T)(implicit ev: T =:= (One :: HNil)) = {}
typed: [T](t: T)(implicit ev: =:=[T,shapeless.::[One,shapeless.HNil]])Unit

scala> typed(1 :: HNil)
<console>:26: error: Cannot prove that shapeless.::[Int,shapeless.HNil] =:= shapeless.::[One,shapeless.HNil].
       typed(1 :: HNil)
            ^

scala> typed(1.narrow :: HNil)

scala>