gc_assign_bug.nim

# compile without (!) `--gc:arc` to get a SIGSEGV 
import typetraits
type
  RawMutableView*[T] = distinct ptr UncheckedArray[T]

  CpuStorage*[T] = ref CpuStorageObj[T]
  CpuStorageObj[T] = object
    raw_buffer*: seq[T]
  Tensor[T] = object
    buf*: CpuStorage[T] # if tensor stores a `seq` itself it works fine

proc allocCpuStorage[T](s: var CpuStorage[T], size: int) =
  new(s)
  s.raw_buffer = newSeq[T](size)

proc newTensor[T](size: int): Tensor[T] =
  allocCpuStorage(result.buf, size)

template `[]=`*[T](v: RawMutableView[T], idx: int, val: T) =
  bind distinctBase
  distinctBase(type v)(v)[idx] = val

proc `[]`[T](t: Tensor[T], idx: int): T = t.buf.raw_buffer[idx]

proc `[]=`[T](t: Tensor[T], idx: int, val: T) =
  var dat: RawMutableView[T]
  dat = cast[RawMutableView[T]](t.buf.raw_buffer[0].addr)
  dat[idx] = val
  # works if we use:
  #t.buf.raw_buffer[idx] = val
  # also works if we manually unref the `dat[idx]`:
  # GC_unref(dat[idx])

proc toTensor[T](s: seq[T]): Tensor[T] =
  result = newTensor[T](s.len)
  for i, x in s:
    result[i] = x

proc main() =
  var s: seq[string]
  for i in 0 ..< 1000: # works for small numbers
    s.add $i
  var y = s.toTensor
  for i in 0 ..< 1000:
    y[i] = "iae " & $i
  echo y.buf.raw_buffer

main()

#[
Traceback (most recent call last)
/home/basti/org/Misc/gc_assign_bug.nim(46) gc_assign_bug
/home/basti/org/Misc/gc_assign_bug.nim(44) main
/home/basti/src/nim/nim_git_repo/lib/system/dollars.nim(170) $
/home/basti/src/nim/nim_git_repo/lib/system/dollars.nim(153) collectionToString
/home/basti/src/nim/nim_git_repo/lib/system.nim(2853) addQuoted
/home/basti/src/nim/nim_git_repo/lib/system.nim(2815) addEscapedChar
/home/basti/src/nim/nim_git_repo/lib/system/gc.nim(435) newObjNoInit
SIGSEGV: Illegal storage access. (Attempt to read from nil?)
Error: execution of an external program failed: '/home/basti/org/Misc/gc_assign_bug '
]#