Compiler crash (null pointer exception) with implicit dispatch

[yallop]

The following program crashes dotc (from the dotty-0.10.0-RC1 release):

object m {
  trait Foo {
    type T[A]
    def bar : (T[Int] => T[Int]) => T[Int => Int]
  }

  def baz (implicit S: Foo, f : S.T[Int] => S.T[Int]) : S.T[Int => Int] =
    S.bar.apply(f)

  def example(implicit s:Foo) : s.T[Int => Int] = {
    baz((x : s.T[Int]) => x)
  }
}

Here's the output:

exception occurred while typechecking minimal.scala
exception occurred while compiling minimal.scala
Exception in thread "main" java.lang.NullPointerException
	at dotty.tools.dotc.typer.ProtoTypes$FunProto.typeOfArg(ProtoTypes.scala:319)
	at dotty.tools.dotc.typer.Applications$ApplyToUntyped.typeOfArg(Applications.scala:706)
	at dotty.tools.dotc.typer.Applications$ApplyToUntyped.typeOfArg(Applications.scala:706)
	at dotty.tools.dotc.typer.Applications$Application.addTyped$2$$anonfun$1(Applications.scala:477)
	at dotty.tools.dotc.core.Decorators$ListDecorator$.loop$1(Decorators.scala:62)
	at dotty.tools.dotc.core.Decorators$ListDecorator$.mapconserve$extension(Decorators.scala:78)
	at dotty.tools.dotc.typer.Applications$Application.matchArgs(Applications.scala:514)
	at dotty.tools.dotc.typer.Applications$Application.init(Applications.scala:296)
	at dotty.tools.dotc.typer.Applications$TypedApply.<init>(Applications.scala:605)
	at dotty.tools.dotc.typer.Applications$ApplyToUntyped.<init>(Applications.scala:703)
	at dotty.tools.dotc.typer.Applications.simpleApply$1(Applications.scala:763)
	at dotty.tools.dotc.typer.Applications.realApply$4$$anonfun$3(Applications.scala:788)
	at dotty.tools.dotc.typer.Typer.tryEither(Typer.scala:2058)
	at dotty.tools.dotc.typer.Applications.realApply$1(Applications.scala:799)
	at dotty.tools.dotc.typer.Applications.typedApply(Applications.scala:834)
	at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:1882)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:1936)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1967)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1979)
	at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:2050)
	at dotty.tools.dotc.typer.Typer.typedBlock(Typer.scala:669)
	at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:1889)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:1936)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1967)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1979)
	at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:2050)
	at dotty.tools.dotc.typer.Typer.typedDefDef(Typer.scala:1489)
	at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:1869)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:1935)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1967)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1979)
	at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:1998)
	at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:2039)
	at dotty.tools.dotc.typer.Typer.typedClassDef(Typer.scala:1593)
	at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:1872)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:1935)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1967)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1979)
	at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:1998)
	at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:2039)
	at dotty.tools.dotc.typer.Typer.typedPackageDef(Typer.scala:1707)
	at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:1914)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:1936)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1967)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1979)
	at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:2050)
	at dotty.tools.dotc.typer.FrontEnd.typeCheck$$anonfun$1(FrontEnd.scala:60)
	at scala.compat.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12)
	at dotty.tools.dotc.typer.FrontEnd.monitor(FrontEnd.scala:34)
	at dotty.tools.dotc.typer.FrontEnd.typeCheck(FrontEnd.scala:64)
	at dotty.tools.dotc.typer.FrontEnd.runOn$$anonfun$2(FrontEnd.scala:88)
	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:388)
	at dotty.tools.dotc.typer.FrontEnd.runOn(FrontEnd.scala:88)
	at dotty.tools.dotc.Run.runPhases$4$$anonfun$4(Run.scala:175)
	at scala.compat.java8.JProcedure1.apply(JProcedure1.java:18)
	at scala.compat.java8.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.IndexedSeqOptimized.foreach(IndexedSeqOptimized.scala:32)
	at scala.collection.IndexedSeqOptimized.foreach$(IndexedSeqOptimized.scala:29)
	at scala.collection.mutable.ArrayOps$ofRef.foreach(ArrayOps.scala:194)
	at dotty.tools.dotc.Run.runPhases$5(Run.scala:187)
	at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:195)
	at scala.compat.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12)
	at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:90)
	at dotty.tools.dotc.Run.compileUnits(Run.scala:202)
	at dotty.tools.dotc.Run.compileSources(Run.scala:137)
	at dotty.tools.dotc.Run.compile(Run.scala:121)
	at dotty.tools.dotc.Driver.doCompile(Driver.scala:31)
	at dotty.tools.dotc.Driver.process(Driver.scala:134)
	at dotty.tools.dotc.Driver.process(Driver.scala:103)
	at dotty.tools.dotc.Driver.process(Driver.scala:115)
	at dotty.tools.dotc.Driver.main(Driver.scala:142)
	at dotty.tools.dotc.Main.main(Main.scala)

[odersky]

Thanks for reporting this, Jeremy!

The fix in #5312 now produces the following error:

11 |    baz((x : s.T[Int]) => x)
   |                           ^
   |                           found:    s.T[Int] => s.T[Int]
   |                           required: m.Foo

I see where you are going with this. I agree that it would be nice to have a way to distinguish explicit from implicit applications, so that the example would typecheck. Do you have some ideas what syntax to use for this? We have discussed several variants, but nothing is implemented yet.