value classes lose type parameter bounds

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The following class compiles without "extends AnyVal", but with it:

final class TraversableOnceOps[+A](val collection: TraversableOnce[A]) extends AnyVal {
  def reduceLeftOption[B >: A](op: (B, A) => B): Option[B] =
    if (collection.isEmpty) None else Some(collection.reduceLeft[B](op))
}
// error: type arguments [B] do not conform to method reduceLeft's type parameter bounds [B >: A]
//            if (collection.isEmpty) None else Some(collection.reduceLeft[B](op))
//                                                                        ^

This is similar to #6034 and will be fixed in the same spot, with more attention to the handling of method type parameters when synthesizing the extension methods.

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Imported From: https://issues.scala-lang.org/browse/SI-6482?orig=1
Reporter: @paulp

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@paulp said:
Fix pending.

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@paulp said:
scala/scala#1468

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@retronym said:
Fixed by: scala/scala#1468

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@adriaanm said:
reopening for 2.10.1-RC1 backport

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@retronym said:
scala/scala#1976