java.lang.UnsupportedOperationException: Position.start on NoPosition

[lancewalton]

reproduction steps

using Scala 2.13.6

case class Foo[T]()

object Foo extends App {
  val foo = Foo[IO[()]]
}

problem

The problem is that () is a value, not a type. the val line should say:

  val foo = Foo[IO[Unit]]()

So it was my dodgy code that caused the problem to exhibit. But I'd expect the compiler to issue an error, not throw an Exception.

[SethTisue]

I get a normal error as expected:

% scalac -version
Scala compiler version 2.13.6 -- Copyright 2002-2021, LAMP/EPFL and Lightbend, Inc.
% scalac S.scala
S.scala:4: error: Illegal literal type (), use Unit instead
  val foo = Foo[IO[()]]
                   ^
1 error

[lancewalton]

Hmmm... odd. I'll investigate some more.

[lancewalton]

OK. I've confirmed that if I do this in a clean project (defining IO as trait IO[_]), I get a compiler error. I'll try to determine why I'm getting the stack trace in my real project...

[som-snytt]

Note trait IO[_] is not legal syntax. scala/scala#9712

I thought () is just the literal type, but I see that exactly a year ago I contributed the exclusion scala/scala#9183

Dotty says

scala> val c = C[IO[()]]()
1 |val c = C[IO[()]]()
  |               ^
  |               '=>' expected, but ']' found

Maybe the underlying issue has to do with incremental compilation and case companions, just a guess.

[SethTisue]

@lancewalton did you have a chance to get back to this?

[lancewalton]

No, sorry. I never got a chance to investigate.