False positive unused:privates for private object

[NthPortal]

Warning for unused:privates yields a false positive when a method of one private object is accessed from an object in the same package but a different file (despite the code compiling and accessing the method fine). For example,

contents of Test.scala:

package com.example

private object Test {
  def foo: String = "foo"
}

contents of Main.scala:

package com.example

object Main {
  def test: String = Test.foo
}

result of scalac -Ywarn-unused:privates Test.scala Main.scala (for both 2.13.0 and 2.12.9):

Test.scala:3: warning: private object Test in package example is never used
private object Test {
               ^
one warning found