regression since 1.4.2: vm crash with lists.SinglyLinkedRing

[timotheecour]

Example

import lists

template fn() =
  var L: SinglyLinkedRing[int]
  L.prepend(4)
  doAssert $L == "[4]" # ok

  L.prepend(4)
  doAssert $L == "[4, 4]" # bug: crash wo stacktrace or error

static: fn() # bug
fn() # ok

Current Output

crashes with no output if running via choosenim nim shim
crashes with segmentation fault (no output) if running without nim shim on devel nim binary or 1.4.2, eg:
[1] 97786 segmentation fault nim r main.nim

Expected Output

works

Additional Information

1.4.2:
[1] 97786 segmentation fault nim r main
devel 5d8f862 1.5.1:
ditto

1.4.0:
works

discovered while reviewing #16362 and was trying to wrap test inside the classic pattern:

template main =
  # all the tests here
main()
static: main()

which is yet another argument while this pattern is useful, as it'd have prevented this regression

[Araq]

which is yet another argument while this pattern is useful, as it'd have prevented this regression

I don't remember anybody disliking this pattern, it's good.

[ringabout]

The regression is introduced by this PR https://github.com/nim-lang/Nim/pull/15724/files