Make :: type alias covariant

[julien-truffaut]

No description provided.

[julien-truffaut]

I just signed the CLA.

[lrytz]

LGTM, I'm curious though if there's a way to observe the difference.

[julien-truffaut]

@lrytz Yes, you can observe it.

class Foo[+A] {
  def nel: ::[A] = ???
}

fails with:

Error:(4, 7) covariant type A occurs in invariant position in type => ::[A] of method nel
  def nel: ::[A] = ???

but if you desalias it, it works fine:

 class Foo[+A] {
  def nel: scala.collection.immutable.::[A] = ???
}

[lrytz]

@julien-truffaut thanks. Could you add this as a test case? For example here: https://github.com/scala/scala/blob/2.13.x/test/junit/scala/PredefTest.scala

[julien-truffaut]

@lrytz Sure, should it be in scala/tree/2.13.x/test/files/pos ? What's the naming convention for the file?

[lrytz]

test/files/pos is good. Thank you!

[lrytz]

Oh, naming convention if we have no ticket, i'm not sure :-) Alternatively, add it to PredefTest.scala (which I liked above)

[SethTisue]

@julien-truffaut does #8651 have implications here?

[julien-truffaut]

@SethTisue Sorry I don't know.