[BUG] Optional[ArcPointer] as Struct member is crashing

[gtvicentini]

Bug description

I'm trying to implement a self-referential Struct using Optional[ArcPointer], since this seems to be the recommended approach while OwnedPointer can't be used with Optional. Mojo is crashing when I define the following Struct:

# f.mojo

from memory import ArcPointer
from collections import Optional

struct S:
    var s: Optional[ArcPointer[S]]

    fn __init__(out self):
        self.s = None

    fn __moveinit__(out self, owned existing: Self):
        self.s = existing.s

def main():
    print("")

> mojo run f.mojo
[34050:1789565:20241226,195419.434048:WARNING crash_report_exception_handler.cc:257] UniversalExceptionRaise: (os/kern) failure (5)
[1]    34048 bus error  mojo run f.mojo

Steps to reproduce

# f.mojo

from memory import ArcPointer
from collections import Optional

struct S:
    var s: Optional[ArcPointer[S]]

    fn __init__(out self):
        self.s = None

    fn __moveinit__(out self, owned existing: Self):
        self.s = existing.s

def main():
    print("")

> mojo run f.mojo
[34050:1789565:20241226,195419.434048:WARNING crash_report_exception_handler.cc:257] UniversalExceptionRaise: (os/kern) failure (5)
[1]    34048 bus error  mojo run f.mojo

System information

- OS: MacOS Sequoia 15.2 (Apple Silicon)
- mojo -v: 24.6.0 (4487cd6e)
- magic -V: magic 0.5.1 - (based on pixi 0.37.0)

[thatstoasty]

I've run into this as well while working on a cli library. I ended up using a ArcPointer[Optional[T]] for a bit, but recently switched to a List[ArcPointer[T]] and just make sure the list is only ever 0 or 1 elements. Not ideal, but works in the interim.