ValDef tree for field no longer carries modifiers (only tracked by symbol)

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Reported in https://groups.google.com/d/msg/scala-internals/35uGsuIEnDQ/cj6AtfbfBAAJ

this is a regression due to #5141. Before, derived trees for valdefs would be rewritten to have their modifiers reflect that the underlying field is private[this]. After the fields rewrite, this is only done for the modifiers tracked by the tree's symbol. When you call untypecheck, the tree's symbol is dropped and thus it forgets that it's supposed to be private[this].

Logged a ticket to be fixed for 2.12.1.

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Imported From: https://issues.scala-lang.org/browse/SI-10009?orig=1
Reporter: @adriaanm
Affected Versions: 2.12.0

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@retronym said:
This affects regular typechecking, not just macros.

class C {
  def c(a: Any, b: Any*) = a
}
object Test {
  new C().c(b = new { val x = 42 }, a = 0)
}

⚡ ~/scala/2.12.0-RC1/bin/scalac sandbox/test.scala
sandbox/test.scala:5: error: x is already defined as value x
  new C().c(b = new { val x = 42 }, a = 0)
                          ^

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@jvican said:
I've also encountered this misbehaviour when porting spores to 2.12. I'm generating new parameters for classes and then untypechecking, as described in https://groups.google.com/forum/#!msg/scala-internals/35uGsuIEnDQ/cj6AtfbfBAAJ. Looking forward to a fix.

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@retronym said:
scala/scala#5554