Incorrect RefChecks in Scala2 mode

[liufengyun]

The following code compiles in Scalac, but Dotty produces an error without message when compiled with -language:Scala2:

// dotc -d out -language:Scala2 examples/matcher.scala
trait Matchers { matchers =>
  private sealed trait Collected extends Product with Serializable
  def foo[T](collected: Collected, xs: scala.collection.GenTraversable[T]): Int  = 3
}
object Matchers extends Matchers

The problem is that RefChecks does not take into acount that Collected is visible in Matchers$ under Scala2 mode.

The missing error message is caused by the incorrect call getMessage instead of toMessage on TypeError.

https://github.com/lampepfl/dotty/blob/6bb20b33db43171d4b0ca4f7bfcbfbf0d73dcc8c/compiler/src/dotty/tools/dotc/typer/RefChecks.scala#L975-L987

[liufengyun]

I've been playing this, it seems there is no easy fix without introducing complexity and bugs just for an incorrect behavior in Scala2.

Given that:

1. Scala2 does the wrong thing
2. There is a simple fix in source code

We close the issue, and the incompatibility is added to https://github.com/lampepfl/dotty/wiki/Known-incompatibilities-with-scalac .