[ARC] Incorrect sinking of an argument?

[ghost]

This code works fine with the default GC but SIGSEGV's with ARC. Found in https://github.com/trustable-code/NiGui.

Example

type
  Control = ref object of RootObj
    fOnMouseButtonDown: MouseButtonProc

  ControlImpl = ref object of Control

  MouseEvent = ref object
    control: Control
    x: int
    y: int
    button: int

  MouseButtonProc = proc(event: MouseEvent)

proc handleMouseButtonDownEvent(control: Control, event: MouseEvent) = 
  let callback = control.fOnMouseButtonDown
  if callback != nil:
    callback(event)

proc pDefaultControlButtonPressSignal(widget: pointer, event: MouseEvent, data: pointer): bool =
  let control = cast[ControlImpl](data)
  let x = event.x.int
  let y = event.y.int
  var evt = new MouseEvent
  evt.control = control
  case event.button
  of 1: evt.button = 1
  of 2: evt.button = 2
  of 3: evt.button = 3
  else: return
  evt.x = x
  evt.y = y

  control.handleMouseButtonDownEvent(evt)

var c: ControlImpl = new ControlImpl

c.fOnMouseButtonDown = proc(event: MouseEvent) = 
  echo "click"

let cptr = cast[pointer](Control(c))
let cimplptr = cast[pointer](c)
let evt = MouseEvent(control: c, button: 3)

discard pDefaultControlButtonPressSignal(cptr, evt, cimplptr)

Current Output

Traceback (most recent call last)
/home/dian/Projects/data/af.nim(45) af
/home/dian/Projects/data/af.nim(34) pDefaultControlButtonPressSignal
/home/dian/Projects/data/af.nim(16) handleMouseButtonDownEvent
SIGSEGV: Illegal storage access. (Attempt to read from nil?)

Expected Output

click

$ nim -v
Nim Compiler Version 1.3.1 [Linux: amd64]
Compiled at 2020-05-05
Copyright (c) 2006-2020 by Andreas Rumpf

git hash: e86a6d24d5ffe43e9dc7f230b80c274167376225
active boot switches: -d:release

[ghost]

Reduced a bit more:

type
  Control = ref object of RootObj
    fOnMouseButtonDown: MouseButtonProc

  MouseEvent = ref object
    control: Control
    button: int

  MouseButtonProc = proc(event: MouseEvent)

proc handleMouseButtonDownEvent(control: Control, event: MouseEvent) = 
  let callback = control.fOnMouseButtonDown
  if callback != nil:
    callback(event)

proc pDefaultControlButtonPressSignal(widget: Control, event: MouseEvent, data: Control): bool =
  let control = data
  var evt = MouseEvent()
  evt.control = control
  case event.button
  of 1: evt.button = 1
  of 2: evt.button = 2
  of 3: evt.button = 3
  # Works with else: discard instead
  else: return

  control.handleMouseButtonDownEvent(evt)

var c = Control()

c.fOnMouseButtonDown = proc(event: MouseEvent) = 
  echo "click"

discard pDefaultControlButtonPressSignal(c, MouseEvent(control: c, button: 3), c)

[ghost]

Simplified even more:

type
  Control = ref object
    x: int

  MouseEvent = ref object
    control: Control
    button: int

proc run(data: Control) =
  var evt = MouseEvent(button: 1)
  evt.control = data
  if evt.button == 1:
    discard
  else: 
    return

  echo data.x

var c = Control()

run(c)