Spurious scala.language.existentials warning triggered by isInstanceOf in LazyCombiner

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The line

if (other.isInstanceOf[LazyCombiner[_, _, _]]) {

triggers this warning:

    scala/src/library/scala/collection/parallel/mutable/LazyCombiner.scala:33:
    warning: the existential type
        scala.collection.parallel.mutable.LazyCombiner[_$1,_$2,_$3] forSome {
          type _$1; type _$2; type _$3 <: scala.collection.generic.Growable[_$1] with scala.collection.generic.Sizing },
    which cannot be expressed by wildcards, should be enabled by making the implicit value scala.language.existentials $
    if (other.isInstanceOf[LazyCombiner[_, _, _]]) {
                       ^

This diagnostic is false and should be fixed.

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Imported From: https://issues.scala-lang.org/browse/SI-7750?orig=1
Reporter: @soc
Affected Versions: 2.11.0-M5

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@gkossakowski said:
The 2.11.2 is out so I'm rescheduling the issue for 2.11.3.

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@retronym said:
Updating fix-by version to 2.11.5.

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@retronym said:
scala/scala#4108