Got ArrayIndexOutOfBoundsException in SpaceLogic instead of type error due to missing argument

[pdferreira]

Seems pretty similar to #13088

Compiler version

3.0.2

Minimized code

import scala.util.parsing.combinator.Parsers

object P extends Parsers

def res: P.ParseResult[Int] = ???
res match { case P.Success(v) => v } // P.Success takes a 2nd argument, but we get a crash instead of a type error

Output (click arrow to expand)

[error] (run-main-0) java.lang.ArrayIndexOutOfBoundsException: Index 1 out of bounds for length 1
[error] java.lang.ArrayIndexOutOfBoundsException: Index 1 out of bounds for length 1
[error]         at dotty.tools.dotc.transform.patmat.SpaceLogic.sub$1(Space.scala:269)
[error]         at dotty.tools.dotc.transform.patmat.SpaceLogic.minus$$anonfun$4(Space.scala:274)
[error]         at scala.runtime.java8.JFunction1$mcZI$sp.apply(JFunction1$mcZI$sp.scala:17)
[error]         at scala.collection.immutable.List.exists(List.scala:395)
[error]         at dotty.tools.dotc.transform.patmat.SpaceLogic.minus(Space.scala:274)
[error]         at dotty.tools.dotc.transform.patmat.SpaceLogic.minus$(Space.scala:78)
[error]         at dotty.tools.dotc.transform.patmat.SpaceEngine.minus(Space.scala:315)
[error]         at dotty.tools.dotc.transform.patmat.SpaceLogic.minus(Space.scala:244)
[error]         at dotty.tools.dotc.transform.patmat.SpaceLogic.minus$(Space.scala:78)
[error]         at dotty.tools.dotc.transform.patmat.SpaceEngine.minus(Space.scala:315)
[error]         at dotty.tools.dotc.transform.patmat.SpaceLogic.minus$$anonfun$2(Space.scala:250)
[error]         at scala.collection.immutable.List.map(List.scala:246)
[error]         at scala.collection.immutable.List.map(List.scala:79)
[error]         at dotty.tools.dotc.transform.patmat.SpaceLogic.minus(Space.scala:250)
[error]         at dotty.tools.dotc.transform.patmat.SpaceLogic.minus$(Space.scala:78)
[error]         at dotty.tools.dotc.transform.patmat.SpaceEngine.minus(Space.scala:315)
[error]         at dotty.tools.dotc.transform.patmat.SpaceLogic.tryDecompose1$3(Space.scala:230)
[error]         at dotty.tools.dotc.transform.patmat.SpaceLogic.minus(Space.scala:246)
[error]         at dotty.tools.dotc.transform.patmat.SpaceLogic.minus$(Space.scala:78)
[error]         at dotty.tools.dotc.transform.patmat.SpaceEngine.minus(Space.scala:315)
[error]         at dotty.tools.dotc.transform.patmat.SpaceLogic.minus$$anonfun$3(Space.scala:252)
[error]         at scala.collection.LinearSeqOps.foldLeft(LinearSeq.scala:169)
[error]         at scala.collection.LinearSeqOps.foldLeft$(LinearSeq.scala:165)
[error]         at scala.collection.immutable.List.foldLeft(List.scala:79)
[error]         at dotty.tools.dotc.transform.patmat.SpaceLogic.minus(Space.scala:252)
[error]         at dotty.tools.dotc.transform.patmat.SpaceLogic.minus$(Space.scala:78)
[error]         at dotty.tools.dotc.transform.patmat.SpaceEngine.minus(Space.scala:315)
[error]         at dotty.tools.dotc.transform.patmat.SpaceEngine.checkExhaustivity(Space.scala:862)
[error]         at dotty.tools.dotc.transform.PatternMatcher.transformMatch(PatternMatcher.scala:45)
[error]         at dotty.tools.dotc.transform.MegaPhase.goMatch(MegaPhase.scala:779)
[error]         at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:369)
[error]         at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:429)
[error]         at dotty.tools.dotc.transform.MegaPhase.mapValDef$1(MegaPhase.scala:235)
[error]         at dotty.tools.dotc.transform.MegaPhase.transformNamed$1(MegaPhase.scala:240)
[error]         at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:427)
[error]         at dotty.tools.dotc.transform.MegaPhase.transformStat$2(MegaPhase.scala:437)
[error]         at dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:442)
[error]         at dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:442)
[error]         at dotty.tools.dotc.transform.MegaPhase.transformStats(MegaPhase.scala:442)
[error]         at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:362)
[error]         at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:429)
[error]         at dotty.tools.dotc.transform.MegaPhase.transformNamed$1(MegaPhase.scala:256)
[error]         at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:427)
[error]         at dotty.tools.dotc.transform.MegaPhase.transformStat$2(MegaPhase.scala:437)
[error]         at dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:442)
[error]         at dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:442)
[error]         at dotty.tools.dotc.transform.MegaPhase.transformStats(MegaPhase.scala:442)
[error]         at dotty.tools.dotc.transform.MegaPhase.mapPackage$1(MegaPhase.scala:382)
[error]         at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:385)
[error]         at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:429)
[error]         at dotty.tools.dotc.transform.MegaPhase.transformUnit(MegaPhase.scala:448)
[error]         at dotty.tools.dotc.transform.MegaPhase.run(MegaPhase.scala:460)
[error]         at dotty.tools.dotc.core.Phases$Phase.runOn$$anonfun$1(Phases.scala:303)
[error]         at scala.collection.immutable.List.map(List.scala:246)
[error]         at dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:304)
[error]         at dotty.tools.dotc.Run.runPhases$4$$anonfun$4(Run.scala:205)
[error]         at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
[error]         at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
[error]         at scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1323)
[error]         at dotty.tools.dotc.Run.runPhases$5(Run.scala:216)
[error]         at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:224)
[error]         at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.scala:18)
[error]         at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:67)
[error]         at dotty.tools.dotc.Run.compileUnits(Run.scala:231)
[error]         at dotty.tools.dotc.Run.compileUnits(Run.scala:172)
[error]         at dotty.tools.repl.ReplCompiler.runCompilationUnit(ReplCompiler.scala:152)
[error]         at dotty.tools.repl.ReplCompiler.compile(ReplCompiler.scala:162)
[error]         at dotty.tools.repl.ReplDriver.compile(ReplDriver.scala:250)
[error]         at dotty.tools.repl.ReplDriver.interpret(ReplDriver.scala:213)
[error]         at dotty.tools.repl.ReplDriver.loop$1(ReplDriver.scala:147)
[error]         at dotty.tools.repl.ReplDriver.runUntilQuit$$anonfun$1(ReplDriver.scala:150)
[error]         at dotty.tools.repl.ReplDriver.withRedirectedOutput(ReplDriver.scala:169)
[error]         at dotty.tools.repl.ReplDriver.runUntilQuit(ReplDriver.scala:150)
[error]         at xsbt.ConsoleInterface.run(ConsoleInterface.java:52)
[error]         at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
[error]         at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
[error]         at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
[error]         at java.base/java.lang.reflect.Method.invoke(Method.java:566)
[error]         at sbt.internal.inc.AnalyzingCompiler.invoke(AnalyzingCompiler.scala:329)
[error]         at sbt.internal.inc.AnalyzingCompiler.console(AnalyzingCompiler.scala:233)
[error]         at sbt.Console.console0$1(Console.scala:64)
[error]         at sbt.Console.$anonfun$apply$5(Console.scala:74)
[error]         at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:23)
[error]         at sbt.util.InterfaceUtil$$anon$1.get(InterfaceUtil.scala:17)
[error]         at sbt.TrapExit$App.run(TrapExit.scala:258)
[error]         at java.base/java.lang.Thread.run(Thread.java:829)

[KacperFKorban]

Minimized it to a single file:

sealed trait Result[+T]

case class Success[+T](result: T, next: Int) extends Result[T] {
  def isEmpty = true
  def get: T = result
}

def main =
  val res: Result[Int] = ???
  res match
    case Success(v) => v

[dwijnand]

Any ideas why this passes typer?

    def main(res: Result[Int]): Int =
      res match
        {
          case Success.unapply[Int](v @ _):Success[Int] =>
            v:Int
        }

[KacperFKorban]

@dwijnand If there is a rule for get match. Should the desired behavior here be, to 'work' instead of throwing an error?
coderel

  /** Does `tp` fit the "get match" conditions as an unapply result type?
   *  This is the case of `tp` has a `get` member as well as a
   *  parameterless `isEmpty` member of result type `Boolean`.
   */
  def isGetMatch(tp: Type, errorPos: SrcPos = NoSourcePosition)(using Context): Boolean =
    extractorMemberType(tp, nme.isEmpty, errorPos).widenSingleton.isRef(defn.BooleanClass) &&
    extractorMemberType(tp, nme.get, errorPos).exists

[dwijnand]

That does seem to be relevant here, because the case class does indeed have an unapply with a result type the satisfies that:

    def unapply[T](x$1: Success[T]): Success[T] = x$1

But the arity of the result type:

    def _1: T = this.result
    def _2: Int = this.next

doesn't match the arity of the extraction, which I would expect the typer to catch. I.e. it shouldn't blow up in SpaceLogic because typer should compile error it already.

[SethTisue]

which I would expect the typer to catch

Scala 2's typer does catch it: error: wrong number of arguments for pattern Success[Int](result: Int, next: Int)

[dwijnand]

Reviewing again I realise now that there's two things at play: "name-based extractions" and "product matches". Name-based extractions were added in 2.11 and require the result to have an isEmpty and a get, i.e. a generalisation of returning Option in an extractor. Product matches are new in 3.0 and generalise case classes extractions, allowing them to unify with how custom extractors were handled (i.e. which is why case classes now synthesises an an unapply which is an identity function).

Seems to me like Typer is accepting the Success result type as a name-based extractor, which returns a single result of type T, instead of considering it a product match, with two components (T and Int). Scala 2 doesn't have product matches so it throws a type error. I think product matches should win (i.e. Typer is wrong), which means Space wouldn't need to change. Here's the doc page on the pattern matching changes:

https://docs.scala-lang.org/scala3/reference/changed-features/pattern-matching.html

When it says:

The former form of unapply has higher precedence, and single match has higher precedence over name-based match.

I think it's saying that product matches (and boolean matches) - aka "the former form" trump name-based matches.

[SethTisue]

Which matches my gut feeling of how it ought to behave.