compiler throws AssertionError when `Product Match` zero-arity

[xuwei-k]

Compiler version

• 3.1.2-RC1-bin-20211115-f215817-NIGHTLY
• 3.1.1-RC1
• 3.1.0
• 3.0.2

Minimized code

class A() extends Product {
  override def canEqual(that: Any) = true
  override def productArity = 0
  override def productElement(n: Int) = null
}

object A {
  def unapply(a: A): A = a
}

object Main {
  (new A) match {
    case A() =>
  }
}

Output

[error] ## Exception when compiling 1 sources to /Users/kenji/product-zero-unapply/target/scala-3.1.1-RC1/classes
[error] java.lang.AssertionError: assertion failed
[error] scala.runtime.Scala3RunTime$.assertFailed(Scala3RunTime.scala:11)
[error] dotty.tools.dotc.transform.PatternMatcher$Translator.unapplyPlan$1$$anonfun$2(PatternMatcher.scala:348)
[error] dotty.tools.dotc.transform.PatternMatcher$Translator.letAbstract(PatternMatcher.scala:107)
[error] dotty.tools.dotc.transform.PatternMatcher$Translator.unapplyPlan$1(PatternMatcher.scala:366)
[error] dotty.tools.dotc.transform.PatternMatcher$Translator.patternPlan(PatternMatcher.scala:401)
[error] dotty.tools.dotc.transform.PatternMatcher$Translator.caseDefPlan(PatternMatcher.scala:437)
[error] dotty.tools.dotc.transform.PatternMatcher$Translator.matchPlan$$anonfun$1$$anonfun$1(PatternMatcher.scala:444)
[error] scala.collection.immutable.List.foldRight(List.scala:352)
[error] dotty.tools.dotc.transform.PatternMatcher$Translator.matchPlan$$anonfun$1(PatternMatcher.scala:444)
[error] dotty.tools.dotc.transform.PatternMatcher$Translator.letAbstract(PatternMatcher.scala:107)
[error] dotty.tools.dotc.transform.PatternMatcher$Translator.matchPlan(PatternMatcher.scala:445)
[error] dotty.tools.dotc.transform.PatternMatcher$Translator.translateMatch(PatternMatcher.scala:1022)
[error] dotty.tools.dotc.transform.PatternMatcher.transformMatch(PatternMatcher.scala:41)
[error] dotty.tools.dotc.transform.MegaPhase.goMatch(MegaPhase.scala:779)
[error] dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:369)
[error] dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:429)
[error] dotty.tools.dotc.transform.MegaPhase.transformStat$1(MegaPhase.scala:439)
[error] dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:442)
[error] dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:442)
[error] dotty.tools.dotc.transform.MegaPhase.transformStats(MegaPhase.scala:442)
[error] dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:362)
[error] dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:429)
[error] dotty.tools.dotc.transform.MegaPhase.transformNamed$1(MegaPhase.scala:256)
[error] dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:427)
[error] dotty.tools.dotc.transform.MegaPhase.transformStat$1(MegaPhase.scala:437)
[error] dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:442)
[error] dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:442)
[error] dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:442)
[error] dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:442)
[error] dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:442)
[error] dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:442)
[error] dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:442)
[error] dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:442)
[error] dotty.tools.dotc.transform.MegaPhase.transformStats(MegaPhase.scala:442)
[error] dotty.tools.dotc.transform.MegaPhase.mapPackage$1(MegaPhase.scala:382)
[error] dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:385)
[error] dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:429)
[error] dotty.tools.dotc.transform.MegaPhase.transformUnit(MegaPhase.scala:448)
[error] dotty.tools.dotc.transform.MegaPhase.run(MegaPhase.scala:460)
[error] dotty.tools.dotc.core.Phases$Phase.runOn$$anonfun$1(Phases.scala:308)
[error] scala.collection.immutable.List.map(List.scala:246)
[error] dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:309)
[error] dotty.tools.dotc.Run.runPhases$1$$anonfun$1(Run.scala:261)
[error] scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
[error] scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
[error] scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1323)
[error] dotty.tools.dotc.Run.runPhases$1(Run.scala:272)
[error] dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:280)
[error] scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.scala:18)
[error] dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:68)
[error] dotty.tools.dotc.Run.compileUnits(Run.scala:289)
[error] dotty.tools.dotc.Run.compileSources(Run.scala:222)
[error] dotty.tools.dotc.Run.compile(Run.scala:206)
[error] dotty.tools.dotc.Driver.doCompile(Driver.scala:39)
[error] dotty.tools.xsbt.CompilerBridgeDriver.run(CompilerBridgeDriver.java:88)
[error] dotty.tools.xsbt.CompilerBridge.run(CompilerBridge.java:22)
[error] sbt.internal.inc.AnalyzingCompiler.compile(AnalyzingCompiler.scala:91)
[error] sbt.internal.inc.MixedAnalyzingCompiler.$anonfun$compile$7(MixedAnalyzingCompiler.scala:192)
[error] scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:23)
[error] sbt.internal.inc.MixedAnalyzingCompiler.timed(MixedAnalyzingCompiler.scala:247)
[error] sbt.internal.inc.MixedAnalyzingCompiler.$anonfun$compile$4(MixedAnalyzingCompiler.scala:182)
[error] sbt.internal.inc.MixedAnalyzingCompiler.$anonfun$compile$4$adapted(MixedAnalyzingCompiler.scala:163)
[error] sbt.internal.inc.JarUtils$.withPreviousJar(JarUtils.scala:239)
[error] sbt.internal.inc.MixedAnalyzingCompiler.compileScala$1(MixedAnalyzingCompiler.scala:163)
[error] sbt.internal.inc.MixedAnalyzingCompiler.compile(MixedAnalyzingCompiler.scala:210)
[error] sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileInternal$1(IncrementalCompilerImpl.scala:528)
[error] sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileInternal$1$adapted(IncrementalCompilerImpl.scala:528)
[error] sbt.internal.inc.Incremental$.$anonfun$apply$5(Incremental.scala:175)
[error] sbt.internal.inc.Incremental$.$anonfun$apply$5$adapted(Incremental.scala:173)
[error] sbt.internal.inc.Incremental$$anon$2.run(Incremental.scala:459)
[error] sbt.internal.inc.IncrementalCommon$CycleState.next(IncrementalCommon.scala:116)
[error] sbt.internal.inc.IncrementalCommon$$anon$1.next(IncrementalCommon.scala:56)
[error] sbt.internal.inc.IncrementalCommon$$anon$1.next(IncrementalCommon.scala:52)
[error] sbt.internal.inc.IncrementalCommon.cycle(IncrementalCommon.scala:263)
[error] sbt.internal.inc.Incremental$.$anonfun$incrementalCompile$8(Incremental.scala:414)
[error] sbt.internal.inc.Incremental$.withClassfileManager(Incremental.scala:501)
[error] sbt.internal.inc.Incremental$.incrementalCompile(Incremental.scala:401)
[error] sbt.internal.inc.Incremental$.apply(Incremental.scala:167)
[error] sbt.internal.inc.IncrementalCompilerImpl.compileInternal(IncrementalCompilerImpl.scala:528)
[error] sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileIncrementally$1(IncrementalCompilerImpl.scala:482)
[error] sbt.internal.inc.IncrementalCompilerImpl.handleCompilationError(IncrementalCompilerImpl.scala:332)
[error] sbt.internal.inc.IncrementalCompilerImpl.compileIncrementally(IncrementalCompilerImpl.scala:420)
[error] sbt.internal.inc.IncrementalCompilerImpl.compile(IncrementalCompilerImpl.scala:137)
[error] sbt.Defaults$.compileIncrementalTaskImpl(Defaults.scala:2357)
[error] sbt.Defaults$.$anonfun$compileIncrementalTask$2(Defaults.scala:2314)
[error] sbt.internal.io.Retry$.apply(Retry.scala:46)
[error] sbt.internal.io.Retry$.apply(Retry.scala:28)
[error] sbt.internal.io.Retry$.apply(Retry.scala:23)
[error] sbt.internal.server.BspCompileTask$.compute(BspCompileTask.scala:31)
[error] sbt.Defaults$.$anonfun$compileIncrementalTask$1(Defaults.scala:2310)
[error] scala.Function1.$anonfun$compose$1(Function1.scala:49)
[error] sbt.internal.util.$tilde$greater.$anonfun$$u2219$1(TypeFunctions.scala:62)
[error] sbt.std.Transform$$anon$4.work(Transform.scala:68)
[error] sbt.Execute.$anonfun$submit$2(Execute.scala:282)
[error] sbt.internal.util.ErrorHandling$.wideConvert(ErrorHandling.scala:23)
[error] sbt.Execute.work(Execute.scala:291)
[error] sbt.Execute.$anonfun$submit$1(Execute.scala:282)
[error] sbt.ConcurrentRestrictions$$anon$4.$anonfun$submitValid$1(ConcurrentRestrictions.scala:265)
[error] sbt.CompletionService$$anon$2.call(CompletionService.scala:64)
[error] java.util.concurrent.FutureTask.run(FutureTask.java:266)
[error] java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
[error] java.util.concurrent.FutureTask.run(FutureTask.java:266)
[error] java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
[error] java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
[error] java.lang.Thread.run(Thread.java:748)

[dwijnand]

See also the variant in #13977, which uses EmptyTuple.

[SethTisue]

It's documented that product matches must be nonzero arity; https://docs.scala-lang.org/scala3/reference/changed-features/pattern-matching.html specifies "N > 0" and did so even when first written (#2565 from 2017).

So it wouldn't be wrong to reject the code (with a proper error message instead of a crash, of course). You can use a boolean match instead of a product match to handle the arity-0 case.

But also, why not simply allow the arity-0 case? It seems harmless to allow it as far as I can see, and allowing it would be more uniform. (And might assist e.g. a code generator, which currently would need to handle the 0 case specially.)

[SethTisue]

#14029 just does the minimal fix to prevent the crash, leaving the question open of whether the 0-arity case could or should be supported in the future.