Dynamic calls get broken by completely unrelated uses of vararg wildcard calls

[scabug]

Take this simple piece of code which works fine:

import scala.language.dynamics

class Foo extends Dynamic {
  def selectDynamic(s: String) {}
}

def bar {
  (new Foo).foo
  //Seq(Seq(): _*)
}

Uncommenting the line with the wildcard vararg call (no matter if it occurs before or after the otherwise working selectDynamic call) makes it fail with the following error:

/Users/szeiger/code/stest/dynamicsvarags.scala:8: error: applyDynamic does not support passing a vararg parameter
  (new Foo).foo
            ^
one error found

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-7420?orig=1
Reporter: @szeiger
Affected Versions: 2.10.6, 2.11.8, 2.12.0-M5

[scabug]

@xeno-by said:
This is a funny consequence of how the typechecker organizes contexts. If we uncomment the provided line, then enclContext.tree, which is used to perform Dynamic desugarings, is going to be the entire block of code, and then the vararg detector that's run on enclosing tree is going to produce a false positive.

Since the entire typer context business is very brittle, I have hard time coming up with an immediate fix here. In the meanwhile, something that might work as a workaround is refactoring the code a bit, so that contexts line up correctly. E.g. writing val temp = (new Foo).foo fixes the problem.

[scabug]

Kazuhiro Sera (seratch) said:
FYI: our library users sometimes suffers #7420 problem.

scalikejdbc/scalikejdbc#253

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Otto Chrons (ochrons) said (edited on Aug 9, 2016 8:40:47 PM UTC):
Ran into this same issue in the ScalaFiddle codebase which is using js.Dynamic. The error triggers even when the vararg call is quite far away from the dynamic usage (in my case updateDynamic)

        editor.xyz = "" // this fails, because of the :_* later
        editor.completers = js.Array(JsVal.obj(
        "getCompletions" -> { (editor: Dyn, session: Dyn, pos: Dyn, prefix: Dyn, callback: Dyn) => {
          Future.successful(Seq(("hauki", "kala"))).map {
            _.map { case (name, value) =>
              JsVal.obj(
                "value" -> value,
                "caption" -> (value + name)
              ).value
            }
          }.foreach { things => callback(null, js.Array(things: _*)) }
        }
        }
      ).value)

[scabug]

@paulp said:
I've also just run into this, again.

import scala.language.dynamics

class ArtifactGroup(org: String, pre: String, rev: String) extends scala.Dynamic {
  def selectDynamic(name: String) = s"$org:$pre-$name:$rev"
}

object Test {
  object scalaz extends ArtifactGroup("org.scalaz", "scalaz", "7.2.4")

  /* ok   */ def f = Seq(scalaz.core, scalaz.effect)
  /* ok   */ def g = Seq(f: _*)
  /* fail */ def h = Seq(Seq(scalaz.core, scalaz.effect): _*)

  // a.scala:12: error: applyDynamic does not support passing a vararg parameter
  //   def h = Seq(Seq(scalaz.core, scalaz.effect): _*)
  //                          ^
}