Compiler crash on irrefutable unapplySeq

[liufengyun]

Discovered when working on #6490

minimized code

final class Foo(val value: Int)

object Foo {
  def unapplySeq(foo: Foo): Seq[Int] = List(foo.value)
}

object Test {
  def main(args: Array[String]): Unit = {
    (new Foo(3)) match {
      case Foo(x, _: _*) =>
        assert(x == 3)
    }
  }
}

Stack trace

exception occurred while compiling tests/run/i6490.scala
java.lang.AssertionError: assertion failed while compiling tests/run/i6490.scala
Exception in thread "main" java.lang.AssertionError: assertion failed
	at dotty.DottyPredef$.assertFail(DottyPredef.scala:15)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.unapplyPlan$3$$anonfun$2(PatternMatcher.scala:332)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.letAbstract(PatternMatcher.scala:100)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.unapplyPlan$1(PatternMatcher.scala:350)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.patternPlan(PatternMatcher.scala:377)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.caseDefPlan(PatternMatcher.scala:413)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.matchPlan$$anonfun$2$$anonfun$1(PatternMatcher.scala:420)
	at scala.collection.immutable.List.$anonfun$foldRight$1(List.scala:408)
	at scala.collection.immutable.List.foldRight(List.scala:89)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.matchPlan$$anonfun$1(PatternMatcher.scala:420)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.letAbstract(PatternMatcher.scala:100)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.matchPlan(PatternMatcher.scala:421)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.translateMatch(PatternMatcher.scala:979)
	at dotty.tools.dotc.transform.PatternMatcher.transformMatch(PatternMatcher.scala:35)
	at dotty.tools.dotc.transform.MegaPhase.goMatch(MegaPhase.scala:746)
	at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:345)
	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:396)
	at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:287)
	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:396)
	at dotty.tools.dotc.transform.MegaPhase.mapDefDef$1(MegaPhase.scala:245)
	at dotty.tools.dotc.transform.MegaPhase.transformNamed$1(MegaPhase.scala:248)
	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:394)
	at dotty.tools.dotc.transform.MegaPhase.transformStat$2(MegaPhase.scala:404)
	at dotty.tools.dotc.transform.MegaPhase.$anonfun$1(MegaPhase.scala:409)
	at scala.collection.immutable.List.mapConserve(List.scala:179)
	at dotty.tools.dotc.transform.MegaPhase.transformStats(MegaPhase.scala:409)
	at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:339)
	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:396)
	at dotty.tools.dotc.transform.MegaPhase.transformNamed$1(MegaPhase.scala:251)
	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:394)
	at dotty.tools.dotc.transform.MegaPhase.transformStat$2(MegaPhase.scala:404)
	at dotty.tools.dotc.transform.MegaPhase.$anonfun$1(MegaPhase.scala:409)
	at scala.collection.immutable.List.mapConserve(List.scala:179)
	at dotty.tools.dotc.transform.MegaPhase.transformStats(MegaPhase.scala:409)
	at dotty.tools.dotc.transform.MegaPhase.mapPackage$1(MegaPhase.scala:356)
	at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:359)
	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:396)
	at dotty.tools.dotc.transform.MegaPhase.transformUnit(MegaPhase.scala:415)
	at dotty.tools.dotc.transform.MegaPhase.run(MegaPhase.scala:427)
	at dotty.tools.dotc.core.Phases$Phase.runOn$$anonfun$1(Phases.scala:316)
	at scala.collection.immutable.List.map(List.scala:286)
	at dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:318)
	at dotty.tools.dotc.Run.runPhases$4$$anonfun$4(Run.scala:158)
	at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.IndexedSeqOptimized.foreach(IndexedSeqOptimized.scala:36)
	at scala.collection.IndexedSeqOptimized.foreach$(IndexedSeqOptimized.scala:33)
	at scala.collection.mutable.ArrayOps$ofRef.foreach(ArrayOps.scala:198)
	at dotty.tools.dotc.Run.runPhases$5(Run.scala:170)
	at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:178)
	at dotty.runtime.function.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12)
	at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:102)
	at dotty.tools.dotc.Run.compileUnits(Run.scala:185)
	at dotty.tools.dotc.Run.compileSources(Run.scala:120)
	at dotty.tools.dotc.Run.compile(Run.scala:104)
	at dotty.tools.dotc.Driver.doCompile(Driver.scala:34)
	at dotty.tools.dotc.Driver.process(Driver.scala:172)
	at dotty.tools.dotc.Driver.process(Driver.scala:141)
	at dotty.tools.dotc.Driver.process(Driver.scala:153)
	at dotty.tools.dotc.Driver.main(Driver.scala:180)
	at dotty.tools.dotc.Main.main(Main.scala)

note

The following code compiles and runs without issue:

final class Foo(val value: Int)

object Foo {
  def unapplySeq(foo: Foo): (Int, Seq[Int]) = (foo.value, Nil)
}

object Test {
  def main(args: Array[String]): Unit = {
    (new Foo(3)) match {
      case Foo(x, _: _*) =>
        assert(x == 3)
    }
  }
}

[sir-wabbit]

A slightly different example of the same bug:

object A {
    def unapplySeq(a: Any): Seq[_] = ""
}
def unapply(x: Any) = x match { case A() => }