Crash on funky EmptyTuple extractor

[LPTK]

Compiler version

3.1.0

Minimized code

class Token
class AST

extension [A] (self: A => Unit)
  def unapply(input: A): EmptyTuple = { self(input); EmptyTuple }

var cur = List.empty[Token]

val accept: List[Token] => Unit =
  cur = _

def block(ts: List[Token]): (AST, List[Token]) =
  cur match
    case t :: accept() => ???
    case _ => ???

https://scastie.scala-lang.org/NYbsLZAiTvCvZ6jyWthm0Q

Output (click arrow to expand)

java.lang.AssertionError: assertion failed
	at scala.runtime.Scala3RunTime$.assertFailed(Scala3RunTime.scala:11)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.unapplyPlan$3$$anonfun$2(PatternMatcher.scala:348)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.letAbstract(PatternMatcher.scala:107)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.unapplyPlan$1(PatternMatcher.scala:366)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.patternPlan(PatternMatcher.scala:401)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.matchArgsPatternPlan$1(PatternMatcher.scala:261)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.matchArgsPatternPlan$1(PatternMatcher.scala:261)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.matchArgsSelectorsPlan$2(PatternMatcher.scala:255)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.matchArgsSelectorsPlan$1$$anonfun$1(PatternMatcher.scala:254)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.letAbstract(PatternMatcher.scala:107)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.matchArgsSelectorsPlan$2(PatternMatcher.scala:254)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.matchArgsSelectorsPlan$1$$anonfun$1(PatternMatcher.scala:254)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.letAbstract(PatternMatcher.scala:107)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.matchArgsSelectorsPlan$2(PatternMatcher.scala:254)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.matchArgsPlan$1(PatternMatcher.scala:266)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.unapplyPlan$1(PatternMatcher.scala:329)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.patternPlan(PatternMatcher.scala:401)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.patternPlan$$anonfun$1(PatternMatcher.scala:381)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.letAbstract(PatternMatcher.scala:107)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.patternPlan(PatternMatcher.scala:381)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.caseDefPlan(PatternMatcher.scala:437)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.matchPlan$$anonfun$2$$anonfun$1(PatternMatcher.scala:444)
	at scala.collection.immutable.List.foldRight(List.scala:352)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.matchPlan$$anonfun$1(PatternMatcher.scala:444)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.letAbstract(PatternMatcher.scala:107)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.matchPlan(PatternMatcher.scala:445)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.translateMatch(PatternMatcher.scala:1019)
	at dotty.tools.dotc.transform.PatternMatcher.transformMatch(PatternMatcher.scala:41)
	at dotty.tools.dotc.transform.MegaPhase.goMatch(MegaPhase.scala:779)
	at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:369)
	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:429)
	at dotty.tools.dotc.transform.MegaPhase.mapDefDef$1(MegaPhase.scala:249)
	at dotty.tools.dotc.transform.MegaPhase.transformNamed$1(MegaPhase.scala:252)
	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:427)
	at dotty.tools.dotc.transform.MegaPhase.transformStat$2(MegaPhase.scala:437)
	at dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:442)
	at dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:442)
	at dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:442)
	at dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:442)
	at dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:442)
	at dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:442)
	at dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:442)
	at dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:442)
	at dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:442)
	at dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:442)
	at dotty.tools.dotc.transform.MegaPhase.transformStats(MegaPhase.scala:442)
	at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:362)
	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:429)
	at dotty.tools.dotc.transform.MegaPhase.transformNamed$1(MegaPhase.scala:256)
	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:427)
	at dotty.tools.dotc.transform.MegaPhase.transformStat$2(MegaPhase.scala:437)
	at dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:442)
	at dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:442)
	at dotty.tools.dotc.transform.MegaPhase.transformStats(MegaPhase.scala:442)
	at dotty.tools.dotc.transform.MegaPhase.mapPackage$1(MegaPhase.scala:382)
	at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:385)
	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:429)
	at dotty.tools.dotc.transform.MegaPhase.transformUnit(MegaPhase.scala:448)
	at dotty.tools.dotc.transform.MegaPhase.run(MegaPhase.scala:460)
	at dotty.tools.dotc.core.Phases$Phase.runOn$$anonfun$1(Phases.scala:308)
	at scala.collection.immutable.List.map(List.scala:246)
	at dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:309)
	at dotty.tools.dotc.Run.runPhases$4$$anonfun$4(Run.scala:261)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1323)
	at dotty.tools.dotc.Run.runPhases$5(Run.scala:272)
	at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:280)
	at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.scala:18)
	at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:68)
	at dotty.tools.dotc.Run.compileUnits(Run.scala:289)
	at dotty.tools.dotc.Run.compileSources(Run.scala:222)
	at dotty.tools.dotc.Run.compile(Run.scala:206)
	at dotty.tools.dotc.Driver.doCompile(Driver.scala:39)
	at dotty.tools.xsbt.CompilerBridgeDriver.run(CompilerBridgeDriver.java:88)
	at dotty.tools.xsbt.CompilerBridge.run(CompilerBridge.java:22)
	at sbt.internal.inc.AnalyzingCompiler.compile(AnalyzingCompiler.scala:91)
	at sbt.internal.inc.MixedAnalyzingCompiler.$anonfun$compile$7(MixedAnalyzingCompiler.scala:192)
	at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:23)
	at sbt.internal.inc.MixedAnalyzingCompiler.timed(MixedAnalyzingCompiler.scala:247)
	at sbt.internal.inc.MixedAnalyzingCompiler.$anonfun$compile$4(MixedAnalyzingCompiler.scala:182)
	at sbt.internal.inc.MixedAnalyzingCompiler.$anonfun$compile$4$adapted(MixedAnalyzingCompiler.scala:163)
	at sbt.internal.inc.JarUtils$.withPreviousJar(JarUtils.scala:239)
	at sbt.internal.inc.MixedAnalyzingCompiler.compileScala$1(MixedAnalyzingCompiler.scala:163)
	at sbt.internal.inc.MixedAnalyzingCompiler.compile(MixedAnalyzingCompiler.scala:210)
	at sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileInternal$1(IncrementalCompilerImpl.scala:528)
	at sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileInternal$1$adapted(IncrementalCompilerImpl.scala:528)
	at sbt.internal.inc.Incremental$.$anonfun$apply$5(Incremental.scala:175)
	at sbt.internal.inc.Incremental$.$anonfun$apply$5$adapted(Incremental.scala:173)
	at sbt.internal.inc.Incremental$$anon$2.run(Incremental.scala:459)
	at sbt.internal.inc.IncrementalCommon$CycleState.next(IncrementalCommon.scala:116)
	at sbt.internal.inc.IncrementalCommon$$anon$1.next(IncrementalCommon.scala:56)
	at sbt.internal.inc.IncrementalCommon$$anon$1.next(IncrementalCommon.scala:52)
	at sbt.internal.inc.IncrementalCommon.cycle(IncrementalCommon.scala:263)
	at sbt.internal.inc.Incremental$.$anonfun$incrementalCompile$8(Incremental.scala:414)
	at sbt.internal.inc.Incremental$.withClassfileManager(Incremental.scala:501)
	at sbt.internal.inc.Incremental$.incrementalCompile(Incremental.scala:401)
	at sbt.internal.inc.Incremental$.apply(Incremental.scala:167)
	at sbt.internal.inc.IncrementalCompilerImpl.compileInternal(IncrementalCompilerImpl.scala:528)
	at sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileIncrementally$1(IncrementalCompilerImpl.scala:482)
	at sbt.internal.inc.IncrementalCompilerImpl.handleCompilationError(IncrementalCompilerImpl.scala:332)
	at sbt.internal.inc.IncrementalCompilerImpl.compileIncrementally(IncrementalCompilerImpl.scala:420)
	at sbt.internal.inc.IncrementalCompilerImpl.compile(IncrementalCompilerImpl.scala:137)
	at sbt.Defaults$.compileIncrementalTaskImpl(Defaults.scala:2357)
	at sbt.Defaults$.$anonfun$compileIncrementalTask$2(Defaults.scala:2314)
	at sbt.internal.io.Retry$.apply(Retry.scala:46)
	at sbt.internal.io.Retry$.apply(Retry.scala:28)
	at sbt.internal.io.Retry$.apply(Retry.scala:23)
	at sbt.internal.server.BspCompileTask$.compute(BspCompileTask.scala:31)
	at sbt.Defaults$.$anonfun$compileIncrementalTask$1(Defaults.scala:2310)
	at scala.Function1.$anonfun$compose$1(Function1.scala:49)
	at sbt.internal.util.$tilde$greater.$anonfun$$u2219$1(TypeFunctions.scala:62)
	at sbt.std.Transform$$anon$4.work(Transform.scala:68)
	at sbt.Execute.$anonfun$submit$2(Execute.scala:282)
	at sbt.internal.util.ErrorHandling$.wideConvert(ErrorHandling.scala:23)
	at sbt.Execute.work(Execute.scala:291)
	at sbt.Execute.$anonfun$submit$1(Execute.scala:282)
	at sbt.ConcurrentRestrictions$$anon$4.$anonfun$submitValid$1(ConcurrentRestrictions.scala:265)
	at sbt.CompletionService$$anon$2.call(CompletionService.scala:64)
	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at java.lang.Thread.run(Thread.java:748)

[nicolasstucki]

Might be related to #13969

[nicolasstucki]

Side note: the code above seems to have a side effects in the unapply. The unapplies are assumed to be pure for optimizations.

[dwijnand]

Duplicate of #13960

[dwijnand]

I'll pull your case, as an interesting variant.

[LPTK]

The unapplies are assumed to be pure for optimizations.

But how can anyone have any fun when all the unapplys are pure? 😞

[dwijnand]

Pedantically (I think I've seen Martin make this point in the past), unapply must be idempotent, not strictly pure.

[LPTK]

Pedantically I'm not sure what idempotent would mean in this context. Calling unapply on the result of unapply does not make much sense. What unapply should arguably be is referentially transparent – so it can use caches, force lazy vals, etc., but in any case it should produce the same results on the same values.

[dwijnand]

Yes, my bad, it must be referentially transparent.