Fix exception on sequence matching with drop

[KavinSatheeskumar]

When using scalac with the flag Ysafe-init-global to compile the following example, I get the following exception. This PR fixes it and adds a test case to catch things like this in the future.

Note, I made this pull request in collaboration with Ondrej Lhotak, Enze Xing, Fengyun Liu and David Hua)

The example

object Matcher {
  val vararg_arr = Array(0, 1, 2, 3)
  val vararg_lst = List(vararg_arr*)
  val vararg_splice = vararg_lst match
    case List(0, 1, xs*) => 1  // binds xs to Seq(2, 3)
    case List(1, _*) => 0           // wildcard pattern
    case _ => 2
  println(vararg_splice)
}

The exception

Exception in thread "main" java.lang.AssertionError: NoDenotation.owner
	at dotty.tools.dotc.core.SymDenotations$NoDenotation$.owner(SymDenotations.scala:2623)
	at dotty.tools.dotc.transform.init.Objects.call(Objects.scala:660)
	at dotty.tools.dotc.transform.init.Objects.evalSeqPatterns$1(Objects.scala:1494)
	at dotty.tools.dotc.transform.init.Objects.evalPattern$1(Objects.scala:1406)
	at dotty.tools.dotc.transform.init.Objects.evalCase$1(Objects.scala:1336)
	at dotty.tools.dotc.transform.init.Objects.patternMatch$$anonfun$1(Objects.scala:1505)
	at scala.collection.immutable.List.map(List.scala:247)
	at dotty.tools.dotc.transform.init.Objects.patternMatch(Objects.scala:1505)
	at dotty.tools.dotc.transform.init.Objects.cases(Objects.scala:1255)
	at dotty.tools.dotc.transform.init.Objects.eval$$anonfun$1(Objects.scala:1095)
	at dotty.tools.dotc.transform.init.Objects$Cache$Data.$anonfun$5(Objects.scala:539)
	at dotty.tools.dotc.transform.init.Cache.cachedEval(Cache.scala:112)
	at dotty.tools.dotc.transform.init.Objects$Cache$Data.cachedEval(Objects.scala:538)
	at dotty.tools.dotc.transform.init.Objects.eval(Objects.scala:1095)
	at dotty.tools.dotc.transform.init.Objects.init$$anonfun$4(Objects.scala:1708)
	at scala.collection.immutable.List.foreach(List.scala:334)
	at dotty.tools.dotc.transform.init.Objects.init(Objects.scala:1705)
	at dotty.tools.dotc.transform.init.Objects$State$.iterate$1(Objects.scala:278)
	at dotty.tools.dotc.transform.init.Objects$State$.doCheckObject(Objects.scala:293)
	at dotty.tools.dotc.transform.init.Objects$State$.checkObjectAccess(Objects.scala:320)
	at dotty.tools.dotc.transform.init.Objects.accessObject(Objects.scala:1059)
	at dotty.tools.dotc.transform.init.Objects.checkClasses$$anonfun$2(Objects.scala:1072)
	at scala.collection.IterableOnceOps.foreach(IterableOnce.scala:619)
	at scala.collection.IterableOnceOps.foreach$(IterableOnce.scala:617)
	at scala.collection.AbstractIterable.foreach(Iterable.scala:935)
	at scala.collection.IterableOps$WithFilter.foreach(Iterable.scala:905)
	at dotty.tools.dotc.transform.init.Objects.checkClasses(Objects.scala:1070)
	at dotty.tools.dotc.transform.init.Checker.runOn$$anonfun$1(Checker.scala:58)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at dotty.tools.dotc.core.Phases$Phase.cancellable(Phases.scala:521)
	at dotty.tools.dotc.transform.init.Checker.runOn(Checker.scala:59)
	at dotty.tools.dotc.Run.runPhases$1$$anonfun$1(Run.scala:343)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1323)
	at dotty.tools.dotc.Run.runPhases$1(Run.scala:336)
	at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:384)
	at dotty.tools.dotc.Run.compileUnits$$anonfun$adapted$1(Run.scala:396)
	at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:69)
	at dotty.tools.dotc.Run.compileUnits(Run.scala:396)
	at dotty.tools.dotc.Run.compileSources(Run.scala:282)
	at dotty.tools.dotc.Run.compile(Run.scala:267)
	at dotty.tools.dotc.Driver.doCompile(Driver.scala:37)
	at dotty.tools.dotc.Driver.process(Driver.scala:201)
	at dotty.tools.dotc.Driver.process(Driver.scala:169)
	at dotty.tools.dotc.Driver.process(Driver.scala:181)
	at dotty.tools.dotc.Driver.main(Driver.scala:211)
	at dotty.tools.dotc.Main.main(Main.scala)

  exception occurred while compiling List(../test-case.scala)

  An unhandled exception was thrown in the compiler.
  Please file a crash report here:
  https://github.com/scala/scala3/issues/new/choose
  For non-enriched exceptions, compile with -Xno-enrich-error-messages.

     while compiling: <no file>
        during phase: parser
                mode: Mode(ImplicitsEnabled,ReadPositions)
     library version: version 2.13.14
    compiler version: version 3.6.0-RC1-bin-SNAPSHOT-nonbootstrapped-git-3dfd762
            settings: -Ysafe-init-global true -classpath /home/kavin/.cache/coursier/v1/https/repo1.maven.org/maven2/org/scala-lang/scala-library/2.13.14/scala-library-2.13.14.jar:/home/kavin/Documents/4A/URA2/scala3/library/../out/bootstrap/scala3-library-bootstrapped/scala-3.6.0-RC1-bin-SNAPSHOT-nonbootstrapped/scala3-library_3-3.6.0-RC1-bin-SNAPSHOT.jar -d /home/kavin/Documents/4A/URA2/scala3/compiler/../out/default-last-scalac-out.jar
[error] Nonzero exit code returned from runner: 1
[error] (scala3-compiler / Compile / runMain) Nonzero exit code returned from runner: 1
[error] Total time: 17 s, completed Jul 26, 2024, 6:35:43 PM

[liufengyun]

LGTM, thank you @KavinSatheeskumar 👍

PS: please rebase against main to avoid unnecessary merge commits.

[liufengyun]

Is this a refactoring or a bug fix? We can remove the commented code.

[KavinSatheeskumar]

This is just a refactor. I forgot to remove it after I tested the code to see if it still worked.

Side note, I'll make sure rebase next time 😅

[liufengyun]

No worries, it's easy to rebase and squash --- you can force push to the PR branch.