Parameter untupling with context function args

[ojow]

Compiler version

3.2.2

Minimized code

type ZZ = String ?=> Int
def f(xs: ZZ*) = xs.zipWithIndex.foreach((f: ZZ, i) => f(using "s"))

Output (click arrow to expand)

[error] scala.MatchError: Block(List(DefDef($anonfun,List(List(ValDef(evidence$1,TypeTree[TypeRef(TermRef(ThisType(TypeRef(NoPrefix,module class scala)),object Predef),type String)],EmptyTree))),TypeTree[TypeRef(TermRef(ThisType(TypeRef(NoPrefix,module class <root>)),object scala),class Int)],Apply(Select(Select(Ident(x$1),_1),apply),List(Ident(evidence$1))))),Closure(List(),Ident($anonfun),EmptyTree)) (of class dotty.tools.dotc.ast.Trees$Block)
[error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.checkStableSelection(PostTyper.scala:268)
[error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform(PostTyper.scala:378)
[error] dotty.tools.dotc.ast.tpd$TreeMapWithPreciseStatContexts.loop$2(tpd.scala:1225)
[error] dotty.tools.dotc.ast.tpd$TreeMapWithPreciseStatContexts.transformStats(tpd.scala:1225)
[error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transformStats(PostTyper.scala:474)
[error] dotty.tools.dotc.ast.tpd$TreeMapWithPreciseStatContexts.transformBlock(tpd.scala:1230)
[error] dotty.tools.dotc.ast.Trees$Instance$TreeMap.transform(Trees.scala:1426)
[error] dotty.tools.dotc.transform.MacroTransform$Transformer.transform(MacroTransform.scala:49)
[error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform(PostTyper.scala:465)
[error] dotty.tools.dotc.ast.Trees$Instance$TreeMap.transform(Trees.scala:1478)
[error] dotty.tools.dotc.transform.MacroTransform$Transformer.transform(MacroTransform.scala:40)
[error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform$$anonfun$5(PostTyper.scala:384)
[error] dotty.tools.dotc.transform.SuperAccessors.wrapDefDef(SuperAccessors.scala:223)
[error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform(PostTyper.scala:384)
[error] dotty.tools.dotc.ast.tpd$TreeMapWithPreciseStatContexts.loop$2(tpd.scala:1225)
[error] dotty.tools.dotc.ast.tpd$TreeMapWithPreciseStatContexts.transformStats(tpd.scala:1225)
[error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transformStats(PostTyper.scala:474)
[error] dotty.tools.dotc.ast.tpd$TreeMapWithPreciseStatContexts.transformBlock(tpd.scala:1230)
[error] dotty.tools.dotc.ast.Trees$Instance$TreeMap.transform(Trees.scala:1426)
[error] dotty.tools.dotc.transform.MacroTransform$Transformer.transform(MacroTransform.scala:49)
[error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform(PostTyper.scala:465)
[error] dotty.tools.dotc.ast.Trees$Instance$TreeMap.transform$$anonfun$1(Trees.scala:1507)
[error] scala.collection.immutable.List.mapConserve(List.scala:472)
[error] dotty.tools.dotc.ast.Trees$Instance$TreeMap.transform(Trees.scala:1507)
[error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.app1$1(PostTyper.scala:323)
[error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform(PostTyper.scala:337)
[error] dotty.tools.dotc.ast.Trees$Instance$TreeMap.transform(Trees.scala:1478)
[error] dotty.tools.dotc.transform.MacroTransform$Transformer.transform(MacroTransform.scala:40)
[error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform$$anonfun$5(PostTyper.scala:384)
[error] dotty.tools.dotc.transform.SuperAccessors.wrapDefDef(SuperAccessors.scala:223)
[error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform(PostTyper.scala:384)
[error] dotty.tools.dotc.ast.tpd$TreeMapWithPreciseStatContexts.loop$2(tpd.scala:1225)
[error] dotty.tools.dotc.ast.tpd$TreeMapWithPreciseStatContexts.transformStats(tpd.scala:1225)
[error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transformStats(PostTyper.scala:474)
[error] dotty.tools.dotc.ast.tpd$TreeMapWithPreciseStatContexts.transformStats(tpd.scala:1227)
[error] dotty.tools.dotc.transform.MacroTransform$Transformer.transform(MacroTransform.scala:47)
[error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform$$anonfun$4$$anonfun$1(PostTyper.scala:371)
[error] dotty.tools.dotc.transform.SuperAccessors.wrapTemplate(SuperAccessors.scala:208)
[error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform$$anonfun$4(PostTyper.scala:371)
[error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.withNoCheckNews(PostTyper.scala:104)
[error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform(PostTyper.scala:373)
[error] dotty.tools.dotc.ast.Trees$Instance$TreeMap.transform(Trees.scala:1480)
[error] dotty.tools.dotc.transform.MacroTransform$Transformer.transform(MacroTransform.scala:40)
[error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform(PostTyper.scala:411)
[error] dotty.tools.dotc.ast.tpd$TreeMapWithPreciseStatContexts.loop$2(tpd.scala:1225)
[error] dotty.tools.dotc.ast.tpd$TreeMapWithPreciseStatContexts.transformStats(tpd.scala:1225)
[error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transformStats(PostTyper.scala:474)
[error] dotty.tools.dotc.ast.tpd$TreeMapWithPreciseStatContexts.transformStats(tpd.scala:1227)
[error] dotty.tools.dotc.ast.Trees$Instance$TreeMap.transform(Trees.scala:1488)
[error] dotty.tools.dotc.transform.MacroTransform$Transformer.transform(MacroTransform.scala:40)
[error] dotty.tools.dotc.transform.PostTyper$PostTyperTransformer.transform(PostTyper.scala:465)
[error] dotty.tools.dotc.transform.MacroTransform.run(MacroTransform.scala:18)
[error] dotty.tools.dotc.core.Phases$Phase.runOn$$anonfun$1(Phases.scala:316)
[error] scala.collection.immutable.List.map(List.scala:246)
[error] dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:320)
[error] dotty.tools.dotc.Run.runPhases$1$$anonfun$1(Run.scala:238)
[error] scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
[error] scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
[error] scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1321)
[error] dotty.tools.dotc.Run.runPhases$1(Run.scala:249)
[error] dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:257)
[error] dotty.tools.dotc.Run.compileUnits$$anonfun$adapted$1(Run.scala:266)
[error] dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:68)
[error] dotty.tools.dotc.Run.compileUnits(Run.scala:266)
[error] dotty.tools.dotc.Run.compileSources(Run.scala:190)
[error] dotty.tools.dotc.Run.compile(Run.scala:174)
[error] dotty.tools.dotc.Driver.doCompile(Driver.scala:35)
[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$7(MixedAnalyzingCompiler.scala:193)
[error] scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:23)
[error] sbt.internal.inc.MixedAnalyzingCompiler.timed(MixedAnalyzingCompiler.scala:248)
[error] sbt.internal.inc.MixedAnalyzingCompiler.$anonfun$compile$4(MixedAnalyzingCompiler.scala:183)
[error] sbt.internal.inc.MixedAnalyzingCompiler.$anonfun$compile$4$adapted(MixedAnalyzingCompiler.scala:163)
[error] sbt.internal.inc.JarUtils$.withPreviousJar(JarUtils.scala:239)
[error] sbt.internal.inc.MixedAnalyzingCompiler.compileScala$1(MixedAnalyzingCompiler.scala:163)
[error] sbt.internal.inc.MixedAnalyzingCompiler.compile(MixedAnalyzingCompiler.scala:211)
[error] sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileInternal$1(IncrementalCompilerImpl.scala:534)
[error] sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileInternal$1$adapted(IncrementalCompilerImpl.scala:534)
[error] sbt.internal.inc.Incremental$.$anonfun$apply$5(Incremental.scala:179)
[error] sbt.internal.inc.Incremental$.$anonfun$apply$5$adapted(Incremental.scala:177)
[error] sbt.internal.inc.Incremental$$anon$2.run(Incremental.scala:463)
[error] sbt.internal.inc.IncrementalCommon$CycleState.next(IncrementalCommon.scala:116)
[error] sbt.internal.inc.IncrementalCommon$$anon$1.next(IncrementalCommon.scala:56)
[error] sbt.internal.inc.IncrementalCommon$$anon$1.next(IncrementalCommon.scala:52)
[error] sbt.internal.inc.IncrementalCommon.cycle(IncrementalCommon.scala:263)
[error] sbt.internal.inc.Incremental$.$anonfun$incrementalCompile$8(Incremental.scala:418)
[error] sbt.internal.inc.Incremental$.withClassfileManager(Incremental.scala:506)
[error] sbt.internal.inc.Incremental$.incrementalCompile(Incremental.scala:405)
[error] sbt.internal.inc.Incremental$.apply(Incremental.scala:171)
[error] sbt.internal.inc.IncrementalCompilerImpl.compileInternal(IncrementalCompilerImpl.scala:534)
[error] sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileIncrementally$1(IncrementalCompilerImpl.scala:488)
[error] sbt.internal.inc.IncrementalCompilerImpl.handleCompilationError(IncrementalCompilerImpl.scala:332)
[error] sbt.internal.inc.IncrementalCompilerImpl.compileIncrementally(IncrementalCompilerImpl.scala:425)
[error] sbt.internal.inc.IncrementalCompilerImpl.compile(IncrementalCompilerImpl.scala:137)
[error] sbt.Defaults$.compileIncrementalTaskImpl(Defaults.scala:2363)
[error] sbt.Defaults$.$anonfun$compileIncrementalTask$2(Defaults.scala:2313)
[error] sbt.internal.server.BspCompileTask$.$anonfun$compute$1(BspCompileTask.scala:30)
[error] sbt.internal.io.Retry$.apply(Retry.scala:46)
[error] sbt.internal.io.Retry$.apply(Retry.scala:28)
[error] sbt.internal.io.Retry$.apply(Retry.scala:23)
[error] sbt.internal.server.BspCompileTask$.compute(BspCompileTask.scala:30)
[error] sbt.Defaults$.$anonfun$compileIncrementalTask$1(Defaults.scala:2311)
[error] scala.Function1.$anonfun$compose$1(Function1.scala:49)
[error] sbt.internal.util.$tilde$greater.$anonfun$$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$2.call(CompletionService.scala:64)
[error] java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
[error] java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)
[error] java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
[error] java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
[error] java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
[error] java.base/java.lang.Thread.run(Thread.java:834)

[Kordyjan]

Still reproducible both on 3.3.0-RC3 and 3.3.1-RC1-bin-20230221-847eccc-NIGHTLY.

[Sporarum]

Stems from two factors:

1. checkStableSelection in PostTyper assumes its parameter will be either _n or apply[T](n)
2. context functions generate useless closures when they are only aliases:

// post typer
type ZZ = String => Int
def f: ZZ = ???
val f2: ZZ = f
// vs
type ZZ = (String) ?=> Int
def f: ZZ =
  {
    def $anonfun(using evidence$1: String): Int = ???
    closure($anonfun)
  }
val f2: ZZ =
  {
    def $anonfun(using evidence$2: String): Int = f.apply(evidence$2)
    closure($anonfun)
  }

Those two clash with each other, as now x$1._1 becomes

{
  def $anonfun(using evidence$1: String): Int =
    x$1._1.apply(evidence$1)
  closure($anonfun)
}

There are thus two ways to solve this problem:

1. Allow checkStableSelection to accept this expanded form
2. Make it so this unneeded expansion is never done

We believe option 2 to be superior, but did not have time to implement it, we can however simplify the test case, now all that remains to test is that

type ZZ = String ?=> Int
def f: ZZ = ???
val f2: ZZ = f

Looks like the following after typer (-Xprint:typer):

package <empty> {
  final lazy module val i16994$package: i16994$package = new i16994$package()
  final module class i16994$package() extends Object() {
    this: i16994$package.type =>
    type ZZ = (String) ?=> Int
    val f: ZZ = ???
    val i: Int = f
  }
}

[Sporarum]

Background information needed:
Context Function:
https://docs.scala-lang.org/scala3/reference/contextual/context-functions.html#
Parameter Untupling:
https://docs.scala-lang.org/scala3/reference/other-new-features/parameter-untupling.html#
And in particular the term adaptation section of the more details page:
https://docs.scala-lang.org/scala3/reference/other-new-features/parameter-untupling-spec.html#term-adaptation

[Sporarum]

Actually, with the reduced test, only knowledge of context functions is needed

[scala-center-bot]

This issue was picked for the Issue Spree No. 28 of 28 March 2023 which takes place in a week from now. @Sporarum, @dwijnand, @yawen-guan will be working on it. If you have any insight into the issue or guidance on how to fix it, please leave it here.

[Sporarum]

I had made a branch for this issue:
https://github.com/dotty-staging/dotty/tree/fix-i16994

[Sporarum]

Maybe we could add something to -Ycheck to make sure the invariant is respected ?