[ARC] SIGSEGV when calling a closure as a tuple field in a seq

[ghost]

This code works on refc, but doesn't work on ARC. As I presume the problem is that for some reason Nim doesn't detect that mCreator is captured by the add and it destroys the closure. Found in https://github.com/ftsf/nimsynth when trying to compile it with ARC :)

Example

type
  Machine = ref object
    hello: string

var machineTypes: seq[tuple[factory: proc(): Machine]]

proc registerMachine(factory: proc(): Machine) =
  var mCreator = proc(): Machine =
    result = factory()

  machineTypes.add((factory: mCreator))

proc facproc(): Machine = 
  result = Machine(hello: "hello")

registerMachine(facproc)

proc createMachine = 
  for machine in machineTypes:
    echo machine.factory().hello
    # different crash with this:
    # echo machine.factory()[]

createMachine()

Current Output

hello
Traceback (most recent call last)
/home/dian/Stuff/arctest/a.nim(5) a
/home/dian/Things/Nim/lib/system/refs_v2.nim(177) nimDestroyAndDispose
/home/dian/Things/Nim/lib/system/refs_v2.nim(162) nimRawDispose
/home/dian/Things/Nim/lib/system/alloc.nim(972) dealloc
/home/dian/Things/Nim/lib/system/alloc.nim(871) rawDealloc
/home/dian/Things/Nim/lib/system/avltree.nim(74) del
SIGSEGV: Illegal storage access. (Attempt to read from nil?)

Expected Output

hello

$ nim -v
Nim Compiler Version 1.3.5 [Linux: amd64]
Compiled at 2020-07-21
Copyright (c) 2006-2020 by Andreas Rumpf

git hash: 911e39351e34c1022b586fe9bfc3a67b2ce3bf44
active boot switches: -d:release -d:danger

[Araq]

@Clyybber this one is for you.

[Araq]

I almost have a fix for this.