undoTo gets head of empty list

[scabug]

Aladdin: [http://scala-webapps.epfl.ch/bugtracking/bugs/displayItem.do?id=1297 bug 1297]

== Code ==

adriaan-moors-computer:~ adriaan$$ ledit ~/src/scala/trunk/build/quick/bin/scala
Welcome to Scala version 2.6.0.12630.20070822-153214.
Type in expressions to have them evaluated.
Type :help for more information.

scala> def foo[A, B, C](l: List[A], f: A => B=>B, g: B=>B=>C): List[C] = l map (g compose f)

== What happened ==

Exception in thread "main" java.util.NoSuchElementException: head of empty list
        at scala.Nil$$.head(List.scala:1193)
        at scala.Nil$$.head(List.scala:1190)
        at scala.tools.nsc.symtab.Types$$class.undoTo(Types.scala:2726)
        at scala.tools.nsc.symtab.Types$$class.isSubType(Types.scala:2888)
        at scala.tools.nsc.symtab.SymbolTable.isSubType(SymbolTable.scala:11)
        at scala.tools.nsc.symtab.Types$$Type.$$less$$colon$$less(Types.scala:469)
        at scala.tools.nsc.symtab.Types$$class.isSubArgs$$0(Types.scala:2950)
        at scala.tools.nsc.symtab.Types$$class.isSubArgs$$0(Types.scala:2951)
        at scala.tools.nsc.symtab.Types$$class.isSubType0(Types.scala:2955)
        at scala.tools.nsc.symtab.Types$$class.isSubType(Types.scala:2886)
        at scala.tools.nsc.symtab.SymbolTable.isSubType(SymbolTable.scala:11)
        at scala.tools.nsc.symtab.Types$$Type.$$less$$colon$$less(Types.scala:469)
        at scala.tools.nsc.typechecker.Infer$$Inferencer.isCompatible(Infer.scala:360)
        at scala.tools.nsc.typechecker.Infer$$Inferencer.isWeaklyCompatible(Infer.scala:364)
        at scala.tools.nsc.typechecker.Infer$$Inferencer.protoTypeArgs(Infer.scala:437)
        at scala.tools.nsc.typechecker.Typers$$Typer.doTypedApply(Typers.scala:1601)
        at scala.tools.nsc.typechecker.Typers$$Typer.typedApply$$0(Typers.scala:2276)
        at scala.tools.nsc.typechecker.Typers$$Typer.typed1(Typers.scala:2836)
        at scala.tools.nsc.typechecker.Typers$$Typer.typed(Typers.scala:2938)
        at scala.tools.nsc.typechecker.Typers$$Typer.typedArg(Typers.scala:1497)
        at scala.tools.nsc.typechecker.Typers$$Typer.typedArgToPoly$$0(Typers.scala:1606)
        at scala.tools.nsc.typechecker.Typers$$Typer$$$$anonfun$$69.apply(Typers.scala:1615)
        at scala.tools.nsc.typechecker.Typers$$Typer$$$$anonfun$$69.apply(Typers.scala:1615)
        at scala.List$$.map2(List.scala:277)
        at scala.tools.nsc.typechecker.Typers$$Typer.doTypedApply(Typers.scala:1615)
        at scala.tools.nsc.typechecker.Typers$$Typer.typedApply$$0(Typers.scala:2276)
        at scala.tools.nsc.typechecker.Typers$$Typer.typed1(Typers.scala:2836)
        at scala.tools.nsc.typechecker.Typers$$Typer.typed(Typers.scala:2938)
        at scala.tools.nsc.typechecker.Typers$$Typer.typed(Typers.scala:2982)
        at scala.tools.nsc.typechecker.Typers$$Typer.transformedOrTyped(Typers.scala:3031)
        at scala.tools.nsc.typechecker.Typers$$Typer.typedDefDef(Typers.scala:1241)
        at scala.tools.nsc.typechecker.Typers$$Typer.typed1(Typers.scala:2679)
        at scala.tools.nsc.typechecker.Typers$$Typer.typed(Typers.scala:2938)
        at scala.tools.nsc.typechecker.Typers$$Typer.typed(Typers.scala:2971)
        at scala.tools.nsc.typechecker.Typers$$Typer.typedStat$$0(Typers.scala:1463)
        at scala.tools.nsc.typechecker.Typers$$Typer$$$$anonfun$$57.apply(Typers.scala:1493)
        at scala.tools.nsc.typechecker.Typers$$Typer$$$$anonfun$$57.apply(Typers.scala:1493)
        at scala.List$$.loop$$0(List.scala:244)
        at scala.List$$.mapConserve(List.scala:261)

What expected

no exception (if I uncomment my change to undoTo, the crash disappears, but it does not type check) Burak pointed out why it doesn't type check.. this works fine (i.e. no type error and no crash):

def foo[A, B, C](l: List[A], f: A => B=>B, g: (B=>B)=>C): List[C] = l map (g compose f)

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-3?orig=1
Reporter: @adriaanm

[scabug]

@odersky said:
Milestone 2.6.0 deleted