Private types trigger false positive with -Ywarn-unused:-privates

[eatkins]

The following code will generate the error "private type Alias in object Foo is never used" in scala 2.12.8:

class Foo
object Foo {
  private type Alias = Foo
  def x: Foo = new Alias
}

This issue also came up in the example provided by @varming in the discussion of
#10313.

[SethTisue]

I'm an unclear on why this should be a separate ticket from #10313?

[som-snytt]

The other example from the other ticket:

scala> object X { private class Y ; class Z extends Y }
                                                    ^
       error: private class Y escapes its defining scope as part of type X.Y

scala> object X { private class Y ; private type YY = Y ; class Z extends YY }
defined object X

I don't know if it's the same symptom. I see YY is unused after uncurry.

Edit: the other ticket was arguably about interaction with macros, but a better mechanism would probably address both bugs.

[eatkins]

I see that @som-snytt already chimed in, but it was his suggestion at the bottom of #10313 that prompted my opening this issue. I have run into the issue multiple times in sbt which has both -Xfatal-warnings and -Ywarn-unused:-private set. I'd have no problem with unifying the two issues either in #10313 itself or a new issue.