Fix #6159: Fix opaque alias

[odersky]

No description provided.

[odersky]

@OlivierBlanvillain ping for review

[OlivierBlanvillain]

The added test case doesn't fail on master (method arguments are just erased differently), so you might want to also include the overloading one:

object O{
  opaque type T[X] = X

  def m(a: T[Int]) = 1
  def m(a: T[String]) = 2
}