Misleading error "recursive functions need annotated return types"

[jiribenes]

Here's the whole error: notice that the real problem is that the actual return type doesn't match the expected return type --- this has nothing to do with recursive / forward-declared functions!

Interestingly, it goes away if we try just functions with numbers. 🤔

Repro:

def foo(): Unit = {
  val x = Nil[Int]()
  head(x)
}

---

Originally reported by @mlutze

[jiribenes]

Fascinatingly, the error doesn't go away if I annotate the return type as Int:

def foo(): Int = {
  val x = Nil[Int]()
  head(x)
}

So this is an actual bug, not just a bad error message -- is it perhaps a "unhandled effect" error? 🤔

[jiribenes]

As @marvinborner noted, this issue is fixed by handling the Exception effect. The following code doesn't report the error anymore:

def foo(): Int = {
  val x = Nil[Int]()
  try {
    head(x)
  } with Exception[EmptyList] {
    def raise(exception: EmptyList, msg: String) = {
      println(msg); 0
    }
  }
}

[jiribenes]

The issue is also fixed by annotating the Exception effect:

def foo(): Int / Exception[EmptyList] = {
  //             ^^^^^^^^^^^^^^^^^^^^
  val x = Nil[Int]()
  head(x)
}

[b-studios]

Strangely it does report the correct error in the following case:

def myHead[A](l: List[A]) = head(l)

def foo(): Unit = {
  val x = Nil[Int]()
  myHead(x)
}