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Crash regarding F-bounded identity alias #20317

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Crash regarding F-bounded identity alias#20317
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area:typeritype:bugitype:crash
@s5bug

Description

@s5bug
s5bug
opened on May 1, 2024

Compiler version

3.4.1, 3.4.2-RC1

Minimized code

type Foo[A] = A

def foo[A <: Foo[A]]: Unit = ()

Output (click arrow to expand)

3.4.2-RC1:

java.lang.AssertionError: assertion failed: NoType
	at scala.runtime.Scala3RunTime$.assertFailed(Scala3RunTime.scala:8)
	at dotty.tools.dotc.core.Types$TypeBounds.&lt;init&gt;(Types.scala:5493)
	at dotty.tools.dotc.core.Types$AliasingBounds.&lt;init&gt;(Types.scala:5572)
	at dotty.tools.dotc.core.Types$TypeAlias.&lt;init&gt;(Types.scala:5596)
	at dotty.tools.dotc.core.Types$TypeAlias$.apply(Types.scala:5633)
	at dotty.tools.dotc.core.Types$Type.bounds(Types.scala:1813)
	at dotty.tools.dotc.core.GadtState.$anonfun$4$$anonfun$1(GadtConstraint.scala:197)
	at scala.collection.immutable.List.map(List.scala:246)
	at dotty.tools.dotc.core.GadtState.$anonfun$4(GadtConstraint.scala:201)
	at dotty.tools.dotc.core.Types$PolyType.&lt;init&gt;(Types.scala:4333)
	at dotty.tools.dotc.core.Types$PolyType$.apply(Types.scala:4439)
	at dotty.tools.dotc.core.GadtState.addToConstraint(GadtConstraint.scala:204)
	at dotty.tools.dotc.core.GadtState.addToConstraint$(GadtConstraint.scala:156)
	at dotty.tools.dotc.core.ProperGadtState.addToConstraint(GadtConstraint.scala:289)
	at dotty.tools.dotc.typer.Typer.typedDefDef(Typer.scala:2619)
	at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:3119)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:3220)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3298)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3302)
	at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:3324)
	at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:3370)
	at dotty.tools.dotc.typer.Typer.typedClassDef(Typer.scala:2814)
	at dotty.tools.dotc.typer.Typer.typedTypeOrClassDef$1(Typer.scala:3125)
	at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:3129)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:3220)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3298)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3302)
	at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:3324)
	at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:3370)
	at dotty.tools.dotc.typer.Typer.typedPackageDef(Typer.scala:2947)
	at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:3171)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:3221)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3298)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3302)
	at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:3413)
	at dotty.tools.dotc.typer.TyperPhase.typeCheck$$anonfun$1(TyperPhase.scala:47)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at dotty.tools.dotc.core.Phases$Phase.monitor(Phases.scala:477)
	at dotty.tools.dotc.typer.TyperPhase.typeCheck(TyperPhase.scala:53)
	at dotty.tools.dotc.typer.TyperPhase.$anonfun$4(TyperPhase.scala:99)
	at scala.collection.Iterator$$anon$6.hasNext(Iterator.scala:479)
	at scala.collection.Iterator$$anon$9.hasNext(Iterator.scala:583)
	at scala.collection.immutable.List.prependedAll(List.scala:152)
	at scala.collection.immutable.List$.from(List.scala:684)
	at scala.collection.immutable.List$.from(List.scala:681)
	at scala.collection.IterableOps$WithFilter.map(Iterable.scala:898)
	at dotty.tools.dotc.typer.TyperPhase.runOn(TyperPhase.scala:100)
	at dotty.tools.dotc.Run.runPhases$1$$anonfun$1(Run.scala:315)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1323)
	at dotty.tools.dotc.Run.runPhases$1(Run.scala:337)
	at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:350)
	at dotty.tools.dotc.Run.compileUnits$$anonfun$adapted$1(Run.scala:360)
	at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:69)
	at dotty.tools.dotc.Run.compileUnits(Run.scala:360)
	at dotty.tools.dotc.Run.compileSources(Run.scala:261)
	at dotty.tools.dotc.Run.compile(Run.scala:246)
	at dotty.tools.dotc.Driver.doCompile(Driver.scala:37)
	at dotty.tools.xsbt.CompilerBridgeDriver.run(CompilerBridgeDriver.java:141)
	at dotty.tools.xsbt.CompilerBridge.run(CompilerBridge.java:22)
	at sbt.internal.inc.AnalyzingCompiler.compile(AnalyzingCompiler.scala:91)
	at sbt.internal.inc.MixedAnalyzingCompiler.$anonfun$compile$7(MixedAnalyzingCompiler.scala:193)
	at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:23)
	at sbt.internal.inc.MixedAnalyzingCompiler.timed(MixedAnalyzingCompiler.scala:248)
	at sbt.internal.inc.MixedAnalyzingCompiler.$anonfun$compile$4(MixedAnalyzingCompiler.scala:183)
	at sbt.internal.inc.MixedAnalyzingCompiler.$anonfun$compile$4$adapted(MixedAnalyzingCompiler.scala:163)
	at sbt.internal.inc.JarUtils$.withPreviousJar(JarUtils.scala:239)
	at sbt.internal.inc.MixedAnalyzingCompiler.compileScala$1(MixedAnalyzingCompiler.scala:163)
	at sbt.internal.inc.MixedAnalyzingCompiler.compile(MixedAnalyzingCompiler.scala:211)
	at sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileInternal$1(IncrementalCompilerImpl.scala:534)
	at sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileInternal$1$adapted(IncrementalCompilerImpl.scala:534)
	at sbt.internal.inc.Incremental$.$anonfun$apply$5(Incremental.scala:180)
	at sbt.internal.inc.Incremental$.$anonfun$apply$5$adapted(Incremental.scala:178)
	at sbt.internal.inc.Incremental$$anon$2.run(Incremental.scala:464)
	at sbt.internal.inc.IncrementalCommon$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$8(Incremental.scala:419)
	at sbt.internal.inc.Incremental$.withClassfileManager(Incremental.scala:506)
	at sbt.internal.inc.Incremental$.incrementalCompile(Incremental.scala:406)
	at sbt.internal.inc.Incremental$.apply(Incremental.scala:172)
	at sbt.internal.inc.IncrementalCompilerImpl.compileInternal(IncrementalCompilerImpl.scala:534)
	at sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileIncrementally$1(IncrementalCompilerImpl.scala:488)
	at sbt.internal.inc.IncrementalCompilerImpl.handleCompilationError(IncrementalCompilerImpl.scala:332)
	at sbt.internal.inc.IncrementalCompilerImpl.compileIncrementally(IncrementalCompilerImpl.scala:425)
	at sbt.internal.inc.IncrementalCompilerImpl.compile(IncrementalCompilerImpl.scala:137)
	at sbt.Defaults$.compileIncrementalTaskImpl(Defaults.scala:2371)
	at sbt.Defaults$.$anonfun$compileIncrementalTask$2(Defaults.scala:2321)
	at sbt.internal.server.BspCompileTask$.$anonfun$compute$1(BspCompileTask.scala:31)
	at sbt.internal.io.Retry$.apply(Retry.scala:47)
	at sbt.internal.io.Retry$.apply(Retry.scala:29)
	at sbt.internal.io.Retry$.apply(Retry.scala:24)
	at sbt.internal.server.BspCompileTask$.compute(BspCompileTask.scala:31)
	at sbt.Defaults$.$anonfun$compileIncrementalTask$1(Defaults.scala:2319)
	at scala.Function1.$anonfun$compose$1(Function1.scala:49)
	at sbt.internal.util.$tilde$greater.$anonfun$$u2219$1(TypeFunctions.scala:63)
	at sbt.std.Transform$$anon$4.work(Transform.scala:69)
	at sbt.Execute.$anonfun$submit$2(Execute.scala:283)
	at sbt.internal.util.ErrorHandling$.wideConvert(ErrorHandling.scala:24)
	at sbt.Execute.work(Execute.scala:292)
	at sbt.Execute.$anonfun$submit$1(Execute.scala:283)
	at sbt.ConcurrentRestrictions$$anon$4.$anonfun$submitValid$1(ConcurrentRestrictions.scala:265)
	at sbt.CompletionService$$anon$2.call(CompletionService.scala:65)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
	at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:539)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)
	at java.base/java.lang.Thread.run(Thread.java:840)

etc

I minimized this from

type SemigroupStructural[A] =
  A & { def combine(a: A): A }
def combineAll[A <: SemigroupStructural[A]](
  i: A, l: List[A]
): A = l.foldLeft(i)(_.combine(_))

which also crashes (but differently, due to a stack overflow) rather than producing a compile error.

Unfortunately I lack scalac on this current computer and I can't figure out how to get Scastie to give me a real stacktrace:

Recursion limit exceeded.
Maybe there is an illegal cyclic reference?
If that's not the case, you could also try to increase the stacksize using the -Xss JVM option.
For the unprocessed stack trace, compile with -Yno-decode-stacktraces.
A recurring operation is (inner to outer):

  subtype (i : A) <:< B
  subtype (i : A) <:< B

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