allow pattern with identifier that would be stable if it wasn't for its volatile type

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https://groups.google.com/forum/?fromgroups=#!topic/scala-internals/COqh09lG0P0

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Imported From: https://issues.scala-lang.org/browse/SI-6815?orig=1
Reporter: @adriaanm

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@retronym said:
scala/scala#2312

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@adriaanm said:
adriaanm/scala@2.10.x...ticket-6815

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@retronym said:
scala/scala#2374