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TypeApplications#noHK: return false for Nothing #966

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@smarter smarter commented Nov 20, 2015

Otherwise, when a higher-kinded type variable is instantiated to Nothing,
we will try to eta-expand it and trigger an assertion in
Definitions#LambdaTrait.

Review by @odersky

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smarter commented Nov 20, 2015

Turns out this does not solve all issues with Nothing as a higher-kinded type, not sure what we should do here without adding even more special cases for Nothing:

object Test {
  def foo[M[_]]: M[Int] = ???
  val x = foo[Nothing]
}
% dotc -Xprint:frontend tests/pos/hk_nothing.scala
result of tests/pos/hk_nothing.scala after frontend:
package <empty> {
  final lazy module val Test: Test$ = new Test$()
  final module class Test$() extends Object() { this: Test.type => 
    def foo[M <: [hk$0] => ]: M[Int] = ???
    val x: Nothing{hk$0 = Int}#$apply = Test.foo[Nothing]
  }
}
dotty.tools.dotc.core.Types$MissingType: cannot resolve reference to type Nothing{hk$0 = Int}.$apply
the classfile defining the type might be missing from the classpath
        at dotty.tools.dotc.core.TypeErasure.dotty$tools$dotc$core$TypeErasure$$sigName(TypeErasure.scala:447)
        at dotty.tools.dotc.core.TypeErasure$.sigName(TypeErasure.scala:134)
        at dotty.tools.dotc.core.Signature$.apply(Signature.scala:76)
        at dotty.tools.dotc.core.Types$MethodicType$class.resultSignature(Types.scala:2047)
        at dotty.tools.dotc.core.Types$ExprType.resultSignature(Types.scala:2238)
        at dotty.tools.dotc.core.Types$ExprType.computeSignature(Types.scala:2242)
        at dotty.tools.dotc.core.Types$MethodicType$class.signature(Types.scala:2057)
        at dotty.tools.dotc.core.Types$ExprType.signature(Types.scala:2238)
        at dotty.tools.dotc.core.Denotations$SingleDenotation.signature(Denotations.scala:460)
        at dotty.tools.dotc.core.SymDenotations$SymDenotation.termRefWithSig(SymDenotations.scala:1060)
        at dotty.tools.dotc.typer.TypeAssigner$class.assignType(TypeAssigner.scala:398)
        at dotty.tools.dotc.typer.Typer.assignType(Typer.scala:57)
        at dotty.tools.dotc.ast.tpd$.polyDefDef(tpd.scala:208)
        at dotty.tools.dotc.ast.tpd$.DefDef(tpd.scala:184)
        at dotty.tools.dotc.ast.tpd$.DefDef(tpd.scala:181)
        at dotty.tools.dotc.transform.Getters.transformValDef(Getters.scala:70)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer.goValDef(TreeTransform.scala:823)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer.transformNamed(TreeTransform.scala:994)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer$$anonfun$transform$2.apply(TreeTransform.scala:1212)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer$$anonfun$transform$2.apply(TreeTransform.scala:1205)
        at dotty.tools.dotc.reporting.Reporting$class.traceIndented(Reporter.scala:154)
        at dotty.tools.dotc.core.Contexts$Context.traceIndented(Contexts.scala:53)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer.transform(TreeTransform.scala:1204)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer.dotty$tools$dotc$transform$TreeTransforms$TreeTransformer$$transformStat$1(TreeTransform.scala:1238)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer$$anonfun$38.apply(TreeTransform.scala:1242)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer$$anonfun$38.apply(TreeTransform.scala:1242)
        at dotty.tools.dotc.core.Decorators$ListDecorator$.loop$1(Decorators.scala:51)
        at dotty.tools.dotc.core.Decorators$ListDecorator$.mapconserve$extension(Decorators.scala:67)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer.transformStats(TreeTransform.scala:1242)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer.transformUnnamed(TreeTransform.scala:1183)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer$$anonfun$transform$2.apply(TreeTransform.scala:1213)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer$$anonfun$transform$2.apply(TreeTransform.scala:1205)
        at dotty.tools.dotc.reporting.Reporting$class.traceIndented(Reporter.scala:154)
        at dotty.tools.dotc.core.Contexts$Context.traceIndented(Contexts.scala:53)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer.transform(TreeTransform.scala:1204)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer.transformNamed(TreeTransform.scala:1011)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer$$anonfun$transform$2.apply(TreeTransform.scala:1212)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer$$anonfun$transform$2.apply(TreeTransform.scala:1205)
        at dotty.tools.dotc.reporting.Reporting$class.traceIndented(Reporter.scala:154)
        at dotty.tools.dotc.core.Contexts$Context.traceIndented(Contexts.scala:53)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer.transform(TreeTransform.scala:1204)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer.dotty$tools$dotc$transform$TreeTransforms$TreeTransformer$$transformStat$1(TreeTransform.scala:1238)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer$$anonfun$38.apply(TreeTransform.scala:1242)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer$$anonfun$38.apply(TreeTransform.scala:1242)
        at dotty.tools.dotc.core.Decorators$ListDecorator$.loop$1(Decorators.scala:51)
        at dotty.tools.dotc.core.Decorators$ListDecorator$.mapconserve$extension(Decorators.scala:67)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer.transformStats(TreeTransform.scala:1242)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer.transformUnnamed(TreeTransform.scala:1192)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer$$anonfun$transform$2.apply(TreeTransform.scala:1213)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer$$anonfun$transform$2.apply(TreeTransform.scala:1205)
        at dotty.tools.dotc.reporting.Reporting$class.traceIndented(Reporter.scala:154)
        at dotty.tools.dotc.core.Contexts$Context.traceIndented(Contexts.scala:53)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer.transform(TreeTransform.scala:1204)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer.macroTransform(TreeTransform.scala:563)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer.run(TreeTransform.scala:480)
        at dotty.tools.dotc.core.Phases$Phase$$anonfun$runOn$1.apply(Phases.scala:271)
        at dotty.tools.dotc.core.Phases$Phase$$anonfun$runOn$1.apply(Phases.scala:269)
        at scala.collection.immutable.List.map(List.scala:273)
        at dotty.tools.dotc.core.Phases$Phase$class.runOn(Phases.scala:269)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer.runOn(TreeTransform.scala:474)
        at dotty.tools.dotc.Run$$anonfun$compileUnits$1$$anonfun$apply$mcV$sp$1.apply(Run.scala:60)
        at dotty.tools.dotc.Run$$anonfun$compileUnits$1$$anonfun$apply$mcV$sp$1.apply(Run.scala:57)
        at scala.collection.IndexedSeqOptimized$class.foreach(IndexedSeqOptimized.scala:33)
        at scala.collection.mutable.ArrayOps$ofRef.foreach(ArrayOps.scala:186)
        at dotty.tools.dotc.Run$$anonfun$compileUnits$1.apply$mcV$sp(Run.scala:57)
        at dotty.tools.dotc.Run$$anonfun$compileUnits$1.apply(Run.scala:52)
        at dotty.tools.dotc.Run$$anonfun$compileUnits$1.apply(Run.scala:52)
        at dotty.tools.dotc.util.Stats$.monitorHeartBeat(Stats.scala:69)
        at dotty.tools.dotc.Run.compileUnits(Run.scala:52)
        at dotty.tools.dotc.Run.compileSources(Run.scala:49)
        at dotty.tools.dotc.Run.compile(Run.scala:33)
        at dotty.tools.dotc.Driver.doCompile(Driver.scala:21)
        at dotty.tools.dotc.Driver.process(Driver.scala:44)
        at dotty.tools.dotc.Driver.process(Driver.scala:50)
        at dotty.tools.dotc.Driver.main(Driver.scala:54)
        at dotty.tools.dotc.Main.main(Main.scala)
cannot take signature of => Nothing{hk$0 = Int}#$apply
exception while transforming val x: Nothing{hk$0 = Int}#$apply = Test.foo[Nothing] of class class dotty.tools.dotc.ast.Trees$ValDef # 135
exception while transforming () extends Object() { this: Test.type => 
  def foo[M <: [hk$0] => ]: M[Int]#$apply = ???
  val x: Nothing{hk$0 = Int}#$apply = Test.foo[Nothing]
} of class class dotty.tools.dotc.ast.Trees$Template # 137
exception while transforming final module class Test$() extends Object() { this: Test.type => 
  def foo[M <: [hk$0] => ]: M[Int]#$apply = ???
  val x: Nothing{hk$0 = Int}#$apply = Test.foo[Nothing]
} of class class dotty.tools.dotc.ast.Trees$TypeDef # 138
exception while transforming package <empty> {
  final lazy module val Test: Test$ = new Test$()
  final module class Test$() extends Object() { this: Test.type => 
    def foo[M <: [hk$0] => ]: M[Int]#$apply = ???
    val x: Nothing{hk$0 = Int}#$apply = Test.foo[Nothing]
  }
} of class class dotty.tools.dotc.ast.Trees$PackageDef # 139
exception occurred while compiling tests/pos/hk_nothing.scala
Exception in thread "main" dotty.tools.dotc.core.Types$MissingType: cannot resolve reference to type Nothing{hk$0 = Int}.$apply
the classfile defining the type might be missing from the classpath
        at dotty.tools.dotc.core.TypeErasure.dotty$tools$dotc$core$TypeErasure$$sigName(TypeErasure.scala:447)
        at dotty.tools.dotc.core.TypeErasure$.sigName(TypeErasure.scala:134)
        at dotty.tools.dotc.core.Signature$.apply(Signature.scala:76)
        at dotty.tools.dotc.core.Types$MethodicType$class.resultSignature(Types.scala:2047)
        at dotty.tools.dotc.core.Types$ExprType.resultSignature(Types.scala:2238)
        at dotty.tools.dotc.core.Types$ExprType.computeSignature(Types.scala:2242)
        at dotty.tools.dotc.core.Types$MethodicType$class.signature(Types.scala:2057)
        at dotty.tools.dotc.core.Types$ExprType.signature(Types.scala:2238)
        at dotty.tools.dotc.core.Denotations$SingleDenotation.signature(Denotations.scala:460)
        at dotty.tools.dotc.core.SymDenotations$SymDenotation.termRefWithSig(SymDenotations.scala:1060)
        at dotty.tools.dotc.typer.TypeAssigner$class.assignType(TypeAssigner.scala:398)
        at dotty.tools.dotc.typer.Typer.assignType(Typer.scala:57)
        at dotty.tools.dotc.ast.tpd$.polyDefDef(tpd.scala:208)
        at dotty.tools.dotc.ast.tpd$.DefDef(tpd.scala:184)
        at dotty.tools.dotc.ast.tpd$.DefDef(tpd.scala:181)
        at dotty.tools.dotc.transform.Getters.transformValDef(Getters.scala:70)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer.goValDef(TreeTransform.scala:823)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer.transformNamed(TreeTransform.scala:994)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer$$anonfun$transform$2.apply(TreeTransform.scala:1212)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer$$anonfun$transform$2.apply(TreeTransform.scala:1205)
        at dotty.tools.dotc.reporting.Reporting$class.traceIndented(Reporter.scala:154)
        at dotty.tools.dotc.core.Contexts$Context.traceIndented(Contexts.scala:53)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer.transform(TreeTransform.scala:1204)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer.dotty$tools$dotc$transform$TreeTransforms$TreeTransformer$$transformStat$1(TreeTransform.scala:1238)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer$$anonfun$38.apply(TreeTransform.scala:1242)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer$$anonfun$38.apply(TreeTransform.scala:1242)
        at dotty.tools.dotc.core.Decorators$ListDecorator$.loop$1(Decorators.scala:51)
        at dotty.tools.dotc.core.Decorators$ListDecorator$.mapconserve$extension(Decorators.scala:67)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer.transformStats(TreeTransform.scala:1242)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer.transformUnnamed(TreeTransform.scala:1183)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer$$anonfun$transform$2.apply(TreeTransform.scala:1213)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer$$anonfun$transform$2.apply(TreeTransform.scala:1205)
        at dotty.tools.dotc.reporting.Reporting$class.traceIndented(Reporter.scala:154)
        at dotty.tools.dotc.core.Contexts$Context.traceIndented(Contexts.scala:53)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer.transform(TreeTransform.scala:1204)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer.transformNamed(TreeTransform.scala:1011)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer$$anonfun$transform$2.apply(TreeTransform.scala:1212)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer$$anonfun$transform$2.apply(TreeTransform.scala:1205)
        at dotty.tools.dotc.reporting.Reporting$class.traceIndented(Reporter.scala:154)
        at dotty.tools.dotc.core.Contexts$Context.traceIndented(Contexts.scala:53)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer.transform(TreeTransform.scala:1204)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer.dotty$tools$dotc$transform$TreeTransforms$TreeTransformer$$transformStat$1(TreeTransform.scala:1238)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer$$anonfun$38.apply(TreeTransform.scala:1242)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer$$anonfun$38.apply(TreeTransform.scala:1242)
        at dotty.tools.dotc.core.Decorators$ListDecorator$.loop$1(Decorators.scala:51)
        at dotty.tools.dotc.core.Decorators$ListDecorator$.mapconserve$extension(Decorators.scala:67)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer.transformStats(TreeTransform.scala:1242)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer.transformUnnamed(TreeTransform.scala:1192)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer$$anonfun$transform$2.apply(TreeTransform.scala:1213)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer$$anonfun$transform$2.apply(TreeTransform.scala:1205)
        at dotty.tools.dotc.reporting.Reporting$class.traceIndented(Reporter.scala:154)
        at dotty.tools.dotc.core.Contexts$Context.traceIndented(Contexts.scala:53)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer.transform(TreeTransform.scala:1204)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer.macroTransform(TreeTransform.scala:563)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer.run(TreeTransform.scala:480)
        at dotty.tools.dotc.core.Phases$Phase$$anonfun$runOn$1.apply(Phases.scala:271)
        at dotty.tools.dotc.core.Phases$Phase$$anonfun$runOn$1.apply(Phases.scala:269)
        at scala.collection.immutable.List.map(List.scala:273)
        at dotty.tools.dotc.core.Phases$Phase$class.runOn(Phases.scala:269)
        at dotty.tools.dotc.transform.TreeTransforms$TreeTransformer.runOn(TreeTransform.scala:474)
        at dotty.tools.dotc.Run$$anonfun$compileUnits$1$$anonfun$apply$mcV$sp$1.apply(Run.scala:60)
        at dotty.tools.dotc.Run$$anonfun$compileUnits$1$$anonfun$apply$mcV$sp$1.apply(Run.scala:57)
        at scala.collection.IndexedSeqOptimized$class.foreach(IndexedSeqOptimized.scala:33)
        at scala.collection.mutable.ArrayOps$ofRef.foreach(ArrayOps.scala:186)
        at dotty.tools.dotc.Run$$anonfun$compileUnits$1.apply$mcV$sp(Run.scala:57)
        at dotty.tools.dotc.Run$$anonfun$compileUnits$1.apply(Run.scala:52)
        at dotty.tools.dotc.Run$$anonfun$compileUnits$1.apply(Run.scala:52)
        at dotty.tools.dotc.util.Stats$.monitorHeartBeat(Stats.scala:69)
        at dotty.tools.dotc.Run.compileUnits(Run.scala:52)
        at dotty.tools.dotc.Run.compileSources(Run.scala:49)
        at dotty.tools.dotc.Run.compile(Run.scala:33)
        at dotty.tools.dotc.Driver.doCompile(Driver.scala:21)
        at dotty.tools.dotc.Driver.process(Driver.scala:44)
        at dotty.tools.dotc.Driver.process(Driver.scala:50)
        at dotty.tools.dotc.Driver.main(Driver.scala:54)
        at dotty.tools.dotc.Main.main(Main.scala)

@smarter
TypeApplications#noHK: return false for Nothing
d6a15c1 
Otherwise, when a higher-kinded type variable is instantiated to Nothing,
we will try to eta-expand it and trigger an assertion in
Definitions#LambdaTrait.
@odersky
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odersky commented Dec 6, 2015

This fails again for the new hk scheme, even after trying an analogous fix. I think it reveals a deeper problem: Nothing is not a legal instance for a higher-kinded type. We need a principled way to avoid this. @smarter Could you investigate this under new scheme please?

@smarter
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smarter commented Dec 6, 2015

I'll take a look.

odersky added a commit to dotty-staging/dotty that referenced this pull request Dec 11, 2015
@odersky
Allow bottom types as hk type arguments
9268573 
Fixes problem raised in scala#966.
@odersky
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odersky commented Dec 11, 2015

Superseded by last commits in #994.

@odersky odersky closed this Dec 11, 2015
odersky added a commit to dotty-staging/dotty that referenced this pull request Dec 11, 2015
@odersky
Allow bottom types as hk type arguments
88f24ef 
    Fixes problem raised in scala#966.
DarkDimius pushed a commit to dotty-linker/dotty that referenced this pull request May 9, 2016
@odersky @DarkDimius
Allow bottom types as hk type arguments
9cae948 
    Fixes problem raised in scala#966.
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@smarter @odersky