Pickling error with match type as method argument type

[milessabin]

Compiling the following,

object Test {
  // The identity on Unit
  type Id[X] = X match {
    case _ => X
  }

  def foo[T](it: Id[T]) = ???
  foo(())
}

with -Xprint-types -Ytest-pickler -Yprint-pos -Yprint-pos-syms as in CompilationTests.pickling results in a round trip failure,

miles@tarski:dotty (master)% diff before-pickling.txt after-pickling.txt
33c33
<       <<Test.foo:([T](it: T): scala.Nothing)>@<118..121>[scala.Unit]:
---
>       <<Test.foo:([T](it: Test.Id[T]): scala.Nothing)>@<118..121>[scala.Unit]:

[odersky]

I believe that's an artifact how things are printed. If a match type has been reduced to some type T we print that type, otherwise we print the match type. Immediately after reading back the match type has not been reduced, hence the difference. To fix this, we'd have to change the printer to not print the reduced type when comparing pickled info.

[milessabin]

Which direction should the change go? Print unreduced before? Or print reduced after?

[smarter]

print unreduced is useful by itself for debugging, so I would go in that direction