Question

I have some Scala code I cannot get to compile, and I simplified it down to what appears to be the essence of the problem.

class Inner[T] {
  class Value
  val values = IndexedSeq.empty[Value]
}

class Outer[T] {
  def inner = new Inner[T]
}

object TestApp {
  def main(args: Array[String]) {
    val outer: Outer[_] = null
    val values = outer.inner.values
    values(0)
  }
}

I am using 2.9.1.final

$ scalac test.scala 
test.scala:14: error: ambiguous reference to overloaded definition,
both method apply in trait SeqLike of type ((idx: Int)Inner[_$1]#Value) forSome { type _$1 }
and  method apply in trait Function1 of type ((v1: Int)Inner[_$1]#Value) forSome { type _$1; type _$1; type _$1 }
match argument types (Int)
    values(0)
    ^
one error found

I am able to make the compilation error go away if I do any of the following:

• Remove inner classes (IndexedSeq.empty[String] instead of IndexedSeq.empty[Value])
• Remove existential types (Outer[String] instead of Outer[_])
• Remove IndexedSeq.apply (values.head instead of values(0))
• Change def inner to val inner (this is the most puzzling one)

Unfortunately in my use case, I can’t change any of those (it’s not evident why in this small example, but the actual code relies on them being that way). Am I doing something forbidden, or is this a limitation of the compiler?

Answer 1

Probably a limitation since it seems fine in 2.10.0-M4.

Of course it gives a NullPointer though since outer is set to null.