Variable with boxed function allows call of function without necessary captures in scope

[dvdvgt]

Consider this program:

interface Greet { def sayHello(): Unit }

def main() = {
  var f = box { () }
  try { f = box { g.sayHello() } }
  with g: Greet { def sayHello() = println("hello!") }
  f()
}

This should of course fail at compile-time since at the call-site of f, the capture g is not in scope. However, it fails with the following error at runtime:

Prompt 2 not found

The type of f is inferred as () => Unit at { g }. Thus, the first assignment is also valid due to capture subtyping. If you actually try to annotate it like this, you cannot since g is not in scope (TIL this is called a Voldemort type according to @jiribenes)

If you instead annotate f with the type of its first assignment, the program expectedly fails at compile-time:

interface Greet { def sayHello(): Unit }

def main() = {
  var f: () => Unit at {} = box { () }
  try { f = box { g.sayHello() } }
  with g: Greet { def sayHello() = println("hello!") }
  f()
}

[b-studios]

Potentially fixed by #552