Typechecker fails when creating an object in array initialization

[EricIO]

The following minimal test case produces the error Cannot typecheck expression new Foo("bug")

class Foo
  name : string
  def init(name : string) : void {
    this.name = name
  }

class Main
  def main() : void {
    let
      arr = [new Foo("bug")]
    in
      print arr
  }

It does work if you create the object first and then initialize the array.

class Main
  def main() : void {
    let
      foo = new Foo("bug")
      arr = [foo]
    in
      print arr
  }

So the problem seems to only appear when you use new in the initialization.

[kaeluka]

if you turn the line: arr = [new Foo("bug")] into arr = [new Foo], it seems to work.

[TobiasWrigstad]

That is not so strange. new Foo("bug") desugars into

let tmp = new Foo in { tmp.init("bug"); tmp }

and we don't have inference working for arbitrary statements, it seems. Or maybe inference is broken in combination with desugaring?

[TobiasWrigstad]

So, I would update the ticket to say that it is not new that is the problem, but constructors.

[EliasC]

My bet is on the desugaring step! The error-message happens because the constructor leaked through to the typechecker, suggesting that desugaring does not check inside arrays. Indeed, src/ir/AST/Util.hs (with which the desugarer is implemented) does not handle arrays at all. getChildren and putChildren need to be updated.

[kaeluka]

If this really is the bug and I'm not mistaken, it could have been caught by exhaustiveness checking: if getChildren and putChildren wouldn't have a catch-all clause at the end, the typechecker would emit a warning every time someone extends the AST but forgets to adapt these functions. This would also mean having to spell out some boring cases explicitly, but the benefits would make up for that, IMO.

[EliasC]

@kaeluka: I agree. I wrote Util.hs and still obviously forgot to add things there, so adding the warning would be doing future implementors and bug-hunters a favor.

[kikofernandez]

closed via PR #147