Flag-related functionality would use a cleanup

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Arguably we need to get rid of flags in Modifiers (similarly to what've been done for Symbols).

However, it'd also be great to support the following use case:

def buildParams(methodType: Type) =
  paramss(methodType) map { params =>
    params map { p =>
      ValDef(
        Modifiers(p.flags),
        newTermName(p.name.toString),
        paramType(p.typeSignature),
        EmptyTree)
    }
  }

Rather controversial requirements, but this only makes it more interesting.

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Imported From: https://issues.scala-lang.org/browse/SI-6267?orig=1
Reporter: @xeno-by
Affected Versions: 2.10.0, 2.11.0

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@xeno-by said:
Also see http://groups.google.com/group/scala-user/browse_thread/thread/b578ef1ce53fbec7

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@xeno-by said:
An interesting idea proposed by Denys is to unify flags and annotations. Under that scheme, PRIVATE would become @Private, and manipulating flags would be as trivial as working with a collection of annotations. Under the covers, noone prevents us from implementing certain system annotations as bits, but users will no longer have to observe this implementation detail.

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@soc said:
Sounds like an interesting approach and would it make easier to make stuff like STATIC or ENUM flags show up without any further intervention!