Wrong type erasure warning with Scala 2.10.0-M2 and -Yvirtpatmat

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When switching from Scala 2.9.1 to 2.10.0-M2 with virtpatmatch enabled, a wrong type erasure warning is emitted in this class: https://github.com/Sciss/ScalaAudioFile/blob/scala-2.10.0-M2-test/src/main/scala/de/sciss/synth/io/AudioFile.scala . The code is as follows:

      val sf   = spec.sampleFormat
      ...
      sf.bidiFactory match {
         case Some( bbf ) =>
            val bb   = bbf( ch, ch, buf, spec.numChannels )
            new BidiFileImpl( f, raf, afh, bb )
         case None        =>
            val bw = sf.writerFactory.map( _.apply( ch, buf, spec.numChannels )).getOrElse( noEncoder( sf ))
            new WritableFileImpl( f, raf, afh, bw )
      }

Method bidiFactory has return type Option[ BufferBidiFactory ], so there is no further generics involved other than having the option. Without virtpatmat it compiles fine, with virtpatmat the following warning is emitted:

[warn] /Users/hhrutz/Documents/devel/ScalaAudioFile/src/main/scala/de/sciss/synth/io/AudioFile.scala:188: non variable type-argument de.sciss.synth.io.BufferBidiFactory in type Some[de.sciss.synth.io.BufferBidiFactory] is unchecked since it is eliminated by erasure
[warn]          case Some( bbf ) =>
[warn]                   ^

Clearly the extractor should work without any erasure warnings. To reproduce clone the following project/branch https://github.com/Sciss/ScalaAudioFile/tree/scala-2.10.0-M2-test and run sbt compile (use the nightly sbt from http://repo.typesafe.com/typesafe/ivy-snapshots/org.scala-tools.sbt/sbt-launch/0.11.3-20120130-052105/ )

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Imported From: https://issues.scala-lang.org/browse/SI-5542?orig=1
Reporter: @Sciss
Affected Versions: 2.10.0-M2

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@SethTisue said (edited on May 23, 2012 1:45:46 PM UTC):
this is reproducible on current master with very simple code:

~/scala % dists/latest/bin/scala -unchecked
Welcome to Scala version 2.10.0-20120424-191812-996f96fb3f

scala> Option(3) match { case Some(n) => n; case None => 0 }
<console>:8: warning: non variable type-argument Int in type Some[Int] is unchecked since it is eliminated by erasure
              Option(3) match { case Some(n) => n; case None => 0 }
                                         ^
res0: Int = 3

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@SethTisue said:
perhaps related: #5503

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@adriaanm said:
fixed by moving patmat to its own phase (the spurious warnings arose from emitted isInstanceOf's, where the unchecked bits are not relied upon)

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@hubertp said:
Test case in eae60ae

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@hubertp said:
Adriaan was quicker: e9ed9d4280246fec.