Referencing to thread argument in a closure causes SIGSEGV (Illegal storage access)

[aj3423]

Nim Version

Nim Compiler Version 2.3.1 [Linux: amd64]
Compiled at 2025-09-19
Copyright (c) 2006-2025 by Andreas Rumpf

git hash: 3f48576
active boot switches: -d:release

Description

The following code crashes:

type 
  Boss = ref object of RootObj
    name: string

proc crash(boss: Boss) =
  (proc () =
    let a = boss.name # using `boss.name` in this closure causes the crash
  )()

proc working(boss: Boss) =
  boss.crash()

let boss = Boss()

var t: Thread[Boss]
createThread(t, working, boss)
joinThread(t)

Passing by pointer also crashes.

Current Output

Traceback (most recent call last)
/run/media/root/e/nim/3/src/test.nim(5) test
/root/.local/share/grabnim/nim-devel/lib/system/orc.nim(527) nimDecRefIsLastCyclicDyn
/root/.local/share/grabnim/nim-devel/lib/system/orc.nim(509) rememberCycle
/root/.local/share/grabnim/nim-devel/lib/system/orc.nim(156) unregisterCycle
SIGSEGV: Illegal storage access. (Attempt to read from nil?)

Expected Output

no crash

Known Workarounds

none

Additional Information

No response