unapplySeq extractor pattern with tuple type won't compile for the empty Seq

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object UnapplySeqTest {
  def unapplySeq(any: Any): Option[(Int, Seq[Int])] = None
}

null match {
  case UnapplySeqTest(5) => println("Matched!")    // shows the error
  case UnapplySeqTest(5, 1) => println("Matched!") // compiles
}

fails to compile with this error:

(the last tuple-component of) the result type of an unapplySeq must be a Seq[_]
  case UnapplySeqTest(5) => println("Matched!")

This is a regression from 2.9.2 probably introduced in ee5721e864.

I tried changing this line to if (productArgs.size == 1) { (v2.10.x branch) because the check should be done on the type of the unapplySeq method IMO. However, this then fails later with this exception:

scala.MatchError: <notype> (of class scala.reflect.internal.Types$NoType$)
	at scala.tools.nsc.typechecker.PatternMatching$MatchTranslation$ExtractorCall.subPatTypes(PatternMatching.scala:642)
	at scala.tools.nsc.typechecker.PatternMatching$MatchTranslation$class.translateExtractorPattern$1(PatternMatching.scala:405)
	at scala.tools.nsc.typechecker.PatternMatching$MatchTranslation$class.translatePattern(PatternMatching.scala:458)
	at scala.tools.nsc.typechecker.PatternMatching$OptimizingMatchTranslator.translatePattern(PatternMatching.scala:132)
	at scala.tools.nsc.typechecker.PatternMatching$MatchTranslation$class.translateCase(PatternMatching.scala:384)
	at scala.tools.nsc.typechecker.PatternMatching$OptimizingMatchTranslator.translateCase(PatternMatching.scala:132)
	at scala.tools.nsc.typechecker.PatternMatching$MatchTranslation$$anonfun$9.apply(PatternMatching.scala:306)
	at scala.tools.nsc.typechecker.PatternMatching$MatchTranslation$$anonfun$9.apply(PatternMatching.scala:306)
	at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:244)
	at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:244)
	at scala.collection.immutable.List.foreach(List.scala:309)
	at scala.collection.TraversableLike$class.map(TraversableLike.scala:244)
	at scala.collection.AbstractTraversable.map(Traversable.scala:105)
	at scala.tools.nsc.typechecker.PatternMatching$MatchTranslation$class.translateMatch(PatternMatching.scala:306)
	at scala.tools.nsc.typechecker.PatternMatching$OptimizingMatchTranslator.translateMatch(PatternMatching.scala:132)
	at scala.tools.nsc.typechecker.PatternMatching$MatchTransformer.transform(PatternMatching.scala:90)
	at scala.tools.nsc.typechecker.PatternMatching$MatchTransformer.transform(PatternMatching.scala:85)
	at scala.reflect.internal.Trees$$anonfun$itransform$1.apply(Trees.scala:1221)
	at scala.reflect.internal.Trees$$anonfun$itransform$1.apply(Trees.scala:1220)
	at scala.reflect.api.Trees$Transformer.atOwner(Trees.scala:2961)

so there seems to be a deeper problem.

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-7039?orig=1
Reporter: @jrudolph
Affected Versions: 2.10.0, 2.11.0-M1

[scabug]

@jrudolph said:
Changing the line from above to if (productArgs.size == 1) to make the error go away in combination with -Xoldpatmat works.

[scabug]

@jrudolph said:
A proposal for a fix:

scala/scala#2003

[scabug]

@retronym said:
Bumping to critical on account of regression status.

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@adriaanm said:
scala/scala#2051