References escape their region when using `var`

[phischu]

Consider the following program where we allocate a mutable variable x in a region r1, and return a closure that contains it.

interface Ref[T] {
  def get(): T
}

def main() = {
  var myref =
    region r1 {
      var x in r1 = "hoho";
      new Ref[String] {
        def get() = x
      }
    }
  println(myref.get())
}

This program is accepted but it should not. When changing var myref to val myref it is rejected, which is good.

[Jonathan Immanuel Brachthäuser]

One bit of additional information: the inferred type of the returned ref is Ref[String] at {r1}, which is correct. So my conjecture is that simply the wellformedness check is buggy:

effekt/effekt/shared/src/main/scala/effekt/typer/Wellformedness.scala

Lines 97 to 109 in 9e65111

 case tree @ source.Region(id, body) => 
   val reg = tree.symbol 
   val tpe = Context.inferredTypeOf(body) 
  
   val free = freeCapture(tpe) 
  
   if (free contains reg.capture) { 
     Context.at(body) { 
       Context.error(pp"The return type ${tpe} of the statement is not allowed to refer to region ${reg}") 
     } 
   } 
  
   scoped { query(body) }