Assertion in TypeErasure.sigName when signature references type coming from self-type

[esarbe]

object Main {

  trait Literal {
    type F[T]
    def num(i: Int): F[Int]
  }

  trait Addition { self: Literal =>
    def add(l: F[Int], r: F[Int]): F[Int]
  }

  def expression(adder: Addition) = {
    import adder._
    add(num(1), num(2))
  }
}

will result in [error] (compile:compileIncremental) java.lang.AssertionError: assertion failed: NoType

Stacktrace:

> compile
[info] Compiling 1 Scala source to /home/raphael/source/final-tagless-encoding/target/scala-0.4/classes...
no sig for NoType because of ()
java.lang.AssertionError: assertion failed: NoType
	at scala.Predef$.assert(Predef.scala:219)
	at dotty.tools.dotc.core.TypeErasure.dotty$tools$dotc$core$TypeErasure$$sigName(TypeErasure.scala:555)
	at dotty.tools.dotc.core.TypeErasure.dotty$tools$dotc$core$TypeErasure$$sigName(TypeErasure.scala:535)
	at dotty.tools.dotc.core.TypeErasure$.sigName(TypeErasure.scala:142)
	at dotty.tools.dotc.core.Signature$.apply(Signature.scala:117)
	at dotty.tools.dotc.core.Types$MethodicType.resultSignature(Types.scala:2473)
	at dotty.tools.dotc.core.Types$MethodType.computeSignature(Types.scala:2715)
	at dotty.tools.dotc.core.Types$MethodicType.signature(Types.scala:2483)
	at dotty.tools.dotc.core.Types$MethodOrPoly.signature(Types.scala:2590)
	at dotty.tools.dotc.core.Denotations$SingleDenotation.signature(Denotations.scala:607)
	at dotty.tools.dotc.core.Types$NamedType.withDenot(Types.scala:1655)
	at dotty.tools.dotc.core.Types$TermRef$.apply(Types.scala:2035)
	at dotty.tools.dotc.core.Types$Type.select(Types.scala:1125)
	at dotty.tools.dotc.typer.Typer.selection$1(Typer.scala:180)
	at dotty.tools.dotc.typer.Typer.wildImportRef$1(Typer.scala:221)
	at dotty.tools.dotc.typer.Typer.loop$2(Typer.scala:312)
	at dotty.tools.dotc.typer.Typer.findRef$1(Typer.scala:331)
	at dotty.tools.dotc.typer.Typer.op$79(Typer.scala:350)
	at dotty.tools.dotc.typer.Typer.typedIdent(Typer.scala:99)
	at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:1612)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:1678)
	at dotty.tools.dotc.typer.Typer.op$8(Typer.scala:1694)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1692)
	at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:1751)
	at dotty.tools.dotc.typer.Applications.op$21(Applications.scala:676)
	at dotty.tools.dotc.typer.Applications.realApply$1(Applications.scala:674)
	at dotty.tools.dotc.typer.Applications.typedApply(Applications.scala:774)
	at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:1632)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:1679)
	at dotty.tools.dotc.typer.Typer.op$8(Typer.scala:1694)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1692)
	at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:1751)
	at dotty.tools.dotc.typer.Typer.op$58(Typer.scala:630)
	at dotty.tools.dotc.typer.Typer.typedBlock(Typer.scala:622)
	at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:1639)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:1679)
	at dotty.tools.dotc.typer.Typer.op$8(Typer.scala:1694)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1692)
	at dotty.tools.dotc.typer.Namer.typedAheadExpr$$anonfun$1(Namer.scala:945)
	at dotty.tools.dotc.typer.Namer.typedAheadImpl(Namer.scala:935)
	at dotty.tools.dotc.typer.Namer.typedAheadExpr(Namer.scala:945)
	at dotty.tools.dotc.typer.Namer.rhsType$1(Namer.scala:1063)
	at dotty.tools.dotc.typer.Namer.cookedRhsType$1(Namer.scala:1064)
	at dotty.tools.dotc.typer.Namer.lhsType$lzyINIT1$1(Namer.scala:1065)
	at dotty.tools.dotc.typer.Namer.lhsType$1(Namer.scala:1065)
	at dotty.tools.dotc.typer.Namer.inferredType$1(Namer.scala:1082)
	at dotty.tools.dotc.typer.Namer.valOrDefDefSig(Namer.scala:1090)
	at dotty.tools.dotc.typer.Namer.defDefSig(Namer.scala:1156)
	at dotty.tools.dotc.typer.Namer$Completer.typeSig(Namer.scala:737)
	at dotty.tools.dotc.typer.Namer$Completer.completeInCreationContext(Namer.scala:782)
	at dotty.tools.dotc.typer.Namer$Completer.complete(Namer.scala:760)
	at dotty.tools.dotc.core.SymDenotations$SymDenotation.completeFrom(SymDenotations.scala:228)
	at dotty.tools.dotc.core.SymDenotations$SymDenotation.info(SymDenotations.scala:206)
	at dotty.tools.dotc.core.SymDenotations$SymDenotation.ensureCompleted(SymDenotations.scala:345)
	at dotty.tools.dotc.typer.Typer.retrieveSym(Typer.scala:1585)
	at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:1610)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:1678)
	at dotty.tools.dotc.typer.Typer.op$8(Typer.scala:1694)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1692)
	at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:1716)
	at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:1739)
	at dotty.tools.dotc.typer.Typer.op$15(Typer.scala:1405)
	at dotty.tools.dotc.typer.Typer.typedClassDef(Typer.scala:1335)
	at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:1623)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:1678)
	at dotty.tools.dotc.typer.Typer.op$8(Typer.scala:1694)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1692)
	at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:1716)
	at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:1739)
	at dotty.tools.dotc.typer.Typer.op$12(Typer.scala:1522)
	at dotty.tools.dotc.typer.Typer.typedPackageDef(Typer.scala:1509)
	at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:1662)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:1679)
	at dotty.tools.dotc.typer.Typer.op$8(Typer.scala:1694)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1692)
	at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:1751)
	at dotty.tools.dotc.typer.FrontEnd.typeCheck$$anonfun$1(FrontEnd.scala:64)
	at scala.compat.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12)
	at dotty.tools.dotc.typer.FrontEnd.monitor(FrontEnd.scala:32)
	at dotty.tools.dotc.typer.FrontEnd.typeCheck(FrontEnd.scala:68)
	at dotty.tools.dotc.typer.FrontEnd.runOn$$anonfun$3(FrontEnd.scala:93)
	at scala.compat.java8.JProcedure1.apply(JProcedure1.java:18)
	at scala.compat.java8.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.immutable.List.foreach(List.scala:389)
	at dotty.tools.dotc.typer.FrontEnd.runOn(FrontEnd.scala:93)
	at dotty.tools.dotc.Run.compileUnits$$anonfun$3$$anonfun$2(Run.scala:123)
	at scala.compat.java8.JProcedure1.apply(JProcedure1.java:18)
	at scala.compat.java8.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.IndexedSeqOptimized.foreach(IndexedSeqOptimized.scala:32)
	at scala.collection.IndexedSeqOptimized.foreach$(IndexedSeqOptimized.scala:29)
	at scala.collection.mutable.ArrayOps$ofRef.foreach(ArrayOps.scala:191)
	at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:131)
	at scala.compat.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12)
	at dotty.tools.dotc.util.Stats$.monitorHeartBeat(Stats.scala:76)
	at dotty.tools.dotc.Run.compileUnits(Run.scala:136)
	at dotty.tools.dotc.Run.compileSources(Run.scala:95)
	at dotty.tools.dotc.Run.compile(Run.scala:79)
	at dotty.tools.dotc.Driver.doCompile(Driver.scala:26)
	at dotty.tools.dotc.Driver.process(Driver.scala:124)
	at xsbt.CachedCompilerImpl.run(CompilerInterface.scala:63)
	at xsbt.CachedCompilerImpl.run(CompilerInterface.scala:53)
	at xsbt.CompilerInterface.run(CompilerInterface.scala:37)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at sbt.compiler.AnalyzingCompiler.call(AnalyzingCompiler.scala:107)
	at sbt.compiler.AnalyzingCompiler.compile(AnalyzingCompiler.scala:53)
	at sbt.compiler.AnalyzingCompiler.compile(AnalyzingCompiler.scala:47)
	at sbt.compiler.MixedAnalyzingCompiler$$anonfun$compileScala$1$1.apply$mcV$sp(MixedAnalyzingCompiler.scala:50)
	at sbt.compiler.MixedAnalyzingCompiler$$anonfun$compileScala$1$1.apply(MixedAnalyzingCompiler.scala:50)
	at sbt.compiler.MixedAnalyzingCompiler$$anonfun$compileScala$1$1.apply(MixedAnalyzingCompiler.scala:50)
	at sbt.compiler.MixedAnalyzingCompiler.timed(MixedAnalyzingCompiler.scala:74)
	at sbt.compiler.MixedAnalyzingCompiler.compileScala$1(MixedAnalyzingCompiler.scala:49)
	at sbt.compiler.MixedAnalyzingCompiler.compile(MixedAnalyzingCompiler.scala:64)
	at sbt.compiler.IC$$anonfun$compileInternal$1.apply(IncrementalCompiler.scala:160)
	at sbt.compiler.IC$$anonfun$compileInternal$1.apply(IncrementalCompiler.scala:160)
	at sbt.inc.IncrementalCompile$$anonfun$doCompile$1.apply(Compile.scala:66)
	at sbt.inc.IncrementalCompile$$anonfun$doCompile$1.apply(Compile.scala:64)
	at sbt.inc.IncrementalCommon.cycle(IncrementalCommon.scala:32)
	at sbt.inc.Incremental$$anonfun$1.apply(Incremental.scala:72)
	at sbt.inc.Incremental$$anonfun$1.apply(Incremental.scala:71)
	at sbt.inc.Incremental$.manageClassfiles(Incremental.scala:99)
	at sbt.inc.Incremental$.compile(Incremental.scala:71)
	at sbt.inc.IncrementalCompile$.apply(Compile.scala:54)
	at sbt.compiler.IC$.compileInternal(IncrementalCompiler.scala:160)
	at sbt.compiler.IC$.incrementalCompile(IncrementalCompiler.scala:138)
	at sbt.Compiler$.compile(Compiler.scala:155)
	at sbt.Compiler$.compile(Compiler.scala:141)
	at sbt.Defaults$.sbt$Defaults$$compileIncrementalTaskImpl(Defaults.scala:886)
	at sbt.Defaults$$anonfun$compileIncrementalTask$1.apply(Defaults.scala:877)
	at sbt.Defaults$$anonfun$compileIncrementalTask$1.apply(Defaults.scala:875)
	at scala.Function1$$anonfun$compose$1.apply(Function1.scala:47)
	at sbt.$tilde$greater$$anonfun$$u2219$1.apply(TypeFunctions.scala:40)
	at sbt.std.Transform$$anon$4.work(System.scala:63)
	at sbt.Execute$$anonfun$submit$1$$anonfun$apply$1.apply(Execute.scala:228)
	at sbt.Execute$$anonfun$submit$1$$anonfun$apply$1.apply(Execute.scala:228)
	at sbt.ErrorHandling$.wideConvert(ErrorHandling.scala:17)
	at sbt.Execute.work(Execute.scala:237)
	at sbt.Execute$$anonfun$submit$1.apply(Execute.scala:228)
	at sbt.Execute$$anonfun$submit$1.apply(Execute.scala:228)
	at sbt.ConcurrentRestrictions$$anon$4$$anonfun$1.apply(ConcurrentRestrictions.scala:159)
	at sbt.CompletionService$$anon$2.call(CompletionService.scala:28)
	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
	at java.util.concurrent.Executors$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)
no sig for HKApply(TypeRef(TermRef(NoPrefix,adder),F),List(TypeRef(TermRef(ThisType(TypeRef(NoPrefix,<root>)),scala)/withSig(Signature(List(),)),Int))) because of ()
failure while taking result signature of (l: adder.F[Int], r: adder.F[Int]): adder.F[Int]: adder.F[Int]
[info] 
[info] cannot take signature of (l: adder.F[Int], r: adder.F[Int]): adder.F[Int]
[info] 
[info] exception occurred while typechecking /home/raphael/source/final-tagless-encoding/src/main/scala/Main.scala
[info] 
[info] exception occurred while compiling /home/raphael/source/final-tagless-encoding/src/main/scala/Main.scala
[trace] Stack trace suppressed: run last compile:compileIncremental for the full output.
[error] (compile:compileIncremental) java.lang.AssertionError: assertion failed: NoType
[error] Total time: 1 s, completed Sep 6, 2017 11:28:30 PM

[smarter]

Minimized example reproducing the crash:

trait Literal {
  type F[T]
}

trait Addition { self: Literal =>
  def foo: F[Int]
}

object Main {
  def expression(adder: Addition) = {
    adder.foo
  }
}

This doesn't crash but fails with an error:

trait Literal {
  type F
}

trait Addition { self: Literal =>
  def foo: F
}

object Main {
  def expression(adder: Addition) = {
    adder.foo
  }
}

11 |    adder.foo
   |    ^^^^^^^^^
   |    cannot resolve reference to type Addition(adder).F
   |    the classfile defining the type might be missing from the classpath

@odersky WDYT? Should signatures referring to types coming from the self-type be prohibited, just like signatures referring to private types?

[odersky]

Should signatures referring to types coming from the self-type be prohibited, just like signatures referring to private types?

I think we should try that. One cannot refer to such a member in an expression from the outside, so we should not allow it in the type either.

[odersky]

Note the examples do not crash anymore, but we get an unexpected error:

-- [E007] Type Mismatch Error: i3083.scala:28:38 -------------------------------
28 |    def expression(adder: Addition) = {
   |                                      ^
   |                                    found:    => adder.F[Int](adder.foo)
   |                                    required: adder.F[Int]
   |                                    
29 |      adder.foo
30 |    }

[odersky]

Here's the test file that produced the error:

object Main {

  trait Literal {
    type F[T]
    def num(i: Int): F[Int]
  }

  trait Addition { self: Literal =>
    def add(l: F[Int], r: F[Int]): F[Int]
  }

  def expression(adder: Addition) = {
    import adder._
    add(num(1), num(2)) // error // error
    }
  }

  object Minimized {
    trait Literal {
      type F[T]
    }

    trait Addition { self: Literal =>
      def foo: F[Int]
    }

    object Main {
      def expression(adder: Addition) = { // error (?)
        adder.foo
      }
    }
  }

[odersky]

One problem is that, if we disallow members like F in types we break lots of code, including nsc.
In fact, nsc's use of the cake pattern relies on this feature everywhere.

One solution would be to disallow references to such members in inferred types. Note that if we add
an explicit return type to expression, we already get an error, and it's the same error in both dotc and nsc:

dotc:

def expression(adder: Addition): adder.F[Int] = { // error (?)
   |                                           ^^^^^^^
   |type `F` is not a member of Minimized.Addition - did you mean `adder.foo`?

nsc:

i3083.scala:28: error: type F is not a member of Minimized.Addition
          def expression(adder: Addition): adder.F[Int] = { // error (?)
                                                 ^

nsc accepts the code but only if the type is inferred, not given. We should avoid behavior like that. So I think the cleanest solution would be to check inferred types for this illegal pattern. This could give a clear error message, unlike the mystifying one we see now.