NPE when inlining a macro that uses a by-name parameter defining a sealed trait

[hmemcpy]

I found an issue when porting a macro to Scala 3 (using 3.0.0-M1). Took me a while to find it (see it on Scastie), but I managed to narrow it down to:

Minimized code

trait Foo {
  inline def foo[A](t: => A): Unit = ()
}

object Bar extends Foo

object Test {
  Bar.foo {
    sealed trait T1
    case object S1 extends T1
  }
}

Attempting to compile this, gives me an NPE with the following stacktrace:

Output (click arrow to expand)

java.lang.NullPointerException
	at dotty.tools.dotc.core.Symbols$Symbol.computeDenot(Symbols.scala:113)
	at dotty.tools.dotc.core.Symbols$Symbol.denot(Symbols.scala:106)
	at dotty.tools.dotc.core.Symbols$Symbol.isType(Symbols.scala:154)
	at dotty.tools.dotc.core.Types$NamedType$.isType(Types.scala:2563)
	at dotty.tools.dotc.core.Types$NamedType$.apply(Types.scala:2567)
	at dotty.tools.dotc.core.Types$Type.select(Types.scala:1455)
	at dotty.tools.dotc.core.Substituters$.substSym(Substituters.scala:83)
	at dotty.tools.dotc.core.Substituters$SubstSymMap.apply(Substituters.scala:181)
	at dotty.tools.dotc.core.Types$TypeMap.op$proxy11$1(Types.scala:5034)
	at dotty.tools.dotc.core.Types$TypeMap.mapArgs(Types.scala:5034)
	at dotty.tools.dotc.core.Types$TypeMap.mapOver(Types.scala:5069)
	at dotty.tools.dotc.core.Substituters$.substSym(Substituters.scala:103)
	at dotty.tools.dotc.core.Types$Type.substSym(Types.scala:1635)
	at dotty.tools.dotc.ast.TreeTypeMap.mapType(TreeTypeMap.scala:63)
	at dotty.tools.dotc.ast.TreeTypeMap.transform(TreeTypeMap.scala:90)
	at dotty.tools.dotc.ast.TreeTypeMap.apply(TreeTypeMap.scala:149)
	at dotty.tools.dotc.ast.TreeTypeMap.apply(TreeTypeMap.scala:151)
	at dotty.tools.dotc.core.Symbols$$anon$2.complete$$anonfun$1(Symbols.scala:820)
	at scala.collection.immutable.List.mapConserve(List.scala:472)
	at dotty.tools.dotc.core.Symbols$$anon$2.complete(Symbols.scala:820)
	at dotty.tools.dotc.core.SymDenotations$SymDenotation.completeFrom(SymDenotations.scala:166)
	at dotty.tools.dotc.core.Denotations$Denotation.completeInfo$1(Denotations.scala:188)
	at dotty.tools.dotc.core.Denotations$Denotation.info(Denotations.scala:190)
	at dotty.tools.dotc.core.SymDenotations$SymDenotation.ensureCompleted(SymDenotations.scala:368)
	at dotty.tools.dotc.core.SymDenotations$ClassDenotation.registeredCompanion(SymDenotations.scala:2147)
	at dotty.tools.dotc.core.Symbols$.mapSymbols$$anonfun$2(Symbols.scala:832)
	at dotty.runtime.function.JProcedure2.apply(JProcedure2.java:15)
	at dotty.runtime.function.JProcedure2.apply(JProcedure2.java:10)
	at scala.collection.LazyZip2.foreach(LazyZipOps.scala:112)
	at dotty.tools.dotc.core.Symbols$.mapSymbols(Symbols.scala:832)
	at dotty.tools.dotc.ast.TreeTypeMap.withMappedSyms(TreeTypeMap.scala:180)
	at dotty.tools.dotc.ast.TreeTypeMap.transformDefs(TreeTypeMap.scala:136)
	at dotty.tools.dotc.ast.TreeTypeMap.transform(TreeTypeMap.scala:107)
	at dotty.tools.dotc.ast.TreeTypeMap.apply(TreeTypeMap.scala:149)
	at dotty.tools.dotc.ast.tpd$TreeOps$.loop$1(tpd.scala:807)
	at dotty.tools.dotc.ast.tpd$TreeOps$.changeOwner$extension(tpd.scala:808)
	at dotty.tools.dotc.typer.Inliner.dotty$tools$dotc$typer$Inliner$$paramBindingDef(Inliner.scala:420)
	at dotty.tools.dotc.typer.Inliner.computeParamBindings$$anonfun$3(Inliner.scala:447)
	at dotty.runtime.function.JProcedure3.apply(JProcedure3.java:15)
	at dotty.runtime.function.JProcedure3.apply(JProcedure3.java:10)
	at scala.collection.LazyZip3.foreach(LazyZipOps.scala:248)
	at dotty.tools.dotc.typer.Inliner.computeParamBindings(Inliner.scala:448)
	at dotty.tools.dotc.typer.Inliner.inlined(Inliner.scala:641)
	at dotty.tools.dotc.typer.Inliner$.inlineCall(Inliner.scala:125)
	at dotty.tools.dotc.typer.Typer.adaptNoArgsOther$4(Typer.scala:3257)
	at dotty.tools.dotc.typer.Typer.adaptNoArgs$1(Typer.scala:3354)
	at dotty.tools.dotc.typer.Typer.adapt1(Typer.scala:3581)
	at dotty.tools.dotc.typer.Typer.adapt(Typer.scala:2920)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2589)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2593)
	at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:2641)
	at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:2664)
	at dotty.tools.dotc.typer.Typer.typedClassDef(Typer.scala:2120)
	at dotty.tools.dotc.typer.Typer.typedTypeOrClassDef$2(Typer.scala:2447)
	at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:2451)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2519)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2589)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2593)
	at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:2615)
	at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:2664)
	at dotty.tools.dotc.typer.Typer.typedPackageDef(Typer.scala:2245)
	at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:2491)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2520)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2589)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2593)
	at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:2641)
	at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:2664)
	at dotty.tools.dotc.typer.Typer.typedPackageDef(Typer.scala:2245)
	at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:2491)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2520)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2589)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2593)
	at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:2708)
	at dotty.tools.dotc.typer.FrontEnd.liftedTree1$1(FrontEnd.scala:79)
	at dotty.tools.dotc.typer.FrontEnd.typeCheck$$anonfun$1(FrontEnd.scala:84)
	at dotty.runtime.function.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12)
	at dotty.tools.dotc.typer.FrontEnd.monitor(FrontEnd.scala:43)
	at dotty.tools.dotc.typer.FrontEnd.typeCheck(FrontEnd.scala:85)
	at dotty.tools.dotc.typer.FrontEnd.runOn$$anonfun$3(FrontEnd.scala:120)
	at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.immutable.List.foreach(List.scala:333)
	at dotty.tools.dotc.typer.FrontEnd.runOn(FrontEnd.scala:120)
	at dotty.tools.dotc.Run.runPhases$4$$anonfun$4(Run.scala:185)
	at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1323)
	at dotty.tools.dotc.Run.runPhases$5(Run.scala:195)
	at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:203)
	at dotty.runtime.function.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12)
	at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:67)
	at dotty.tools.dotc.Run.compileUnits(Run.scala:210)
	at dotty.tools.dotc.Run.compileSources(Run.scala:147)
	at dotty.tools.dotc.Run.compile(Run.scala:129)
	at dotty.tools.dotc.Driver.doCompile(Driver.scala:38)
	at dotty.tools.dotc.Driver.process(Driver.scala:194)
	at dotty.tools.dotc.Main.process(Main.scala)
	at xsbt.CachedCompilerImpl.run(CachedCompilerImpl.java:69)
	at xsbt.CompilerInterface.run(CompilerInterface.java:41)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:566)
	at sbt.internal.inc.AnalyzingCompiler.invoke(AnalyzingCompiler.scala:330)
	at sbt.internal.inc.AnalyzingCompiler.compile(AnalyzingCompiler.scala:113)
	at sbt.internal.inc.MixedAnalyzingCompiler.$anonfun$compile$7(MixedAnalyzingCompiler.scala:186)
	at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:23)
	at sbt.internal.inc.MixedAnalyzingCompiler.timed(MixedAnalyzingCompiler.scala:241)
	at sbt.internal.inc.MixedAnalyzingCompiler.$anonfun$compile$4(MixedAnalyzingCompiler.scala:176)
	at sbt.internal.inc.MixedAnalyzingCompiler.$anonfun$compile$4$adapted(MixedAnalyzingCompiler.scala:157)
	at sbt.internal.inc.JarUtils$.withPreviousJar(JarUtils.scala:239)
	at sbt.internal.inc.MixedAnalyzingCompiler.compileScala$1(MixedAnalyzingCompiler.scala:157)
	at sbt.internal.inc.MixedAnalyzingCompiler.compile(MixedAnalyzingCompiler.scala:204)
	at sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileInternal$1(IncrementalCompilerImpl.scala:573)
	at sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileInternal$1$adapted(IncrementalCompilerImpl.scala:573)
	at sbt.internal.inc.Incremental$.$anonfun$apply$5(Incremental.scala:173)
	at sbt.internal.inc.Incremental$.$anonfun$apply$5$adapted(Incremental.scala:171)
	at sbt.internal.inc.Incremental$$anon$2.run(Incremental.scala:458)
	at sbt.internal.inc.IncrementalCommon$CycleState.next(IncrementalCommon.scala:116)
	at sbt.internal.inc.IncrementalCommon$$anon$1.next(IncrementalCommon.scala:56)
	at sbt.internal.inc.IncrementalCommon$$anon$1.next(IncrementalCommon.scala:52)
	at sbt.internal.inc.IncrementalCommon.cycle(IncrementalCommon.scala:261)
	at sbt.internal.inc.Incremental$.$anonfun$incrementalCompile$8(Incremental.scala:413)
	at sbt.internal.inc.Incremental$.withClassfileManager(Incremental.scala:498)
	at sbt.internal.inc.Incremental$.incrementalCompile(Incremental.scala:400)
	at sbt.internal.inc.Incremental$.apply(Incremental.scala:165)
	at sbt.internal.inc.IncrementalCompilerImpl.compileInternal(IncrementalCompilerImpl.scala:573)
	at sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileIncrementally$1(IncrementalCompilerImpl.scala:491)
	at sbt.internal.inc.IncrementalCompilerImpl.handleCompilationError(IncrementalCompilerImpl.scala:332)
	at sbt.internal.inc.IncrementalCompilerImpl.compileIncrementally(IncrementalCompilerImpl.scala:420)
	at sbt.internal.inc.IncrementalCompilerImpl.compile(IncrementalCompilerImpl.scala:137)
	at sbt.Defaults$.compileIncrementalTaskImpl(Defaults.scala:2176)
	at sbt.Defaults$.$anonfun$compileIncrementalTask$2(Defaults.scala:2133)
	at sbt.internal.io.Retry$.apply(Retry.scala:40)
	at sbt.internal.io.Retry$.apply(Retry.scala:23)
	at sbt.internal.server.BspCompileTask$.compute(BspCompileTask.scala:31)
	at sbt.Defaults$.$anonfun$compileIncrementalTask$1(Defaults.scala:2129)
	at scala.Function1.$anonfun$compose$1(Function1.scala:49)
	at sbt.internal.util.$tilde$greater.$anonfun$$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$2.call(CompletionService.scala:64)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
	at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
	at java.base/java.lang.Thread.run(Thread.java:834)

Removing the sealed keyword does not cause the issue. Changing the by-name to a lambda () => A also does not cause the issue.

Finally, this might be related somewhat to #10540 which I reported earlier, it's a similar test defining some traits inside a body that is used as a by-name parameter.

[griggt]

Possibly related to #9655 / #9572

[hmemcpy]

@griggt Ah, looks very similar indeed!
Edit: the original issue #9572 seems to have a workaround by @nicolasstucki (dotty-staging@01abc11) that makes it work by not inlining the body parameter. However, in my case it's a by-name, and it happens without the inline keyword.

[odersky]

The root cause of all this is that we currently cannot reliably copy non-trivial class hierarchies, and inlining implies copying. To fix this we'd have to look at TreeTypeMap and mapSymbols. This is extremely subtle code that is more easily wrecked than fixed. (Analogous functionality never worked remotely correctly over the lifetime of Scala 2). But somebody will have to take it on to plunge into this and fix it.

[smarter]

I just closed #10540 as a duplicate of this one, the example and crash are different but the root cause is likely the same:

Minimized code

trait Foo {
  inline def foo[A](t: => A): Unit = ()
}

object Bar extends Foo

object Test {
  Bar.foo {
    trait T1
    val array = Array(new T1 {})
  }
}

Output (click arrow to expand)

Scastie output:

java.lang.AssertionError: assertion failed: unresolved symbols: �[33mtrait�[0m �[35mT1�[0m(line 8) when pickling /tmp/scastie7347053754411828221/src/main/scala/main.scala
	at dotty.DottyPredef$.assertFail(DottyPredef.scala:17)
	at dotty.tools.dotc.core.tasty.TreePickler.pickle(TreePickler.scala:765)
	at dotty.tools.dotc.transform.Pickler.run$$anonfun$3$$anonfun$2(Pickler.scala:69)
	at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.immutable.List.foreach(List.scala:333)
	at dotty.tools.dotc.transform.Pickler.run$$anonfun$1(Pickler.scala:105)
        ...

[hmemcpy]

Thanks! Just to emphasize, the assertion error here only happens when using an Array, just something to keep in mind.