-.nim

type
  TTest* = object
    pos*: range[0..15]

proc next(state: var TTest): int64 =
  assert(state.pos >= 0 and state.pos < 16, "pos is " & $state.pos)
  # Error: unhandled exception: state.pos >= 0 and state.pos < 16 pos is 140736480769200 [EAssertionFailed]
  # 140736480769200 is incremented by a few million each execution
  state.pos = (state.pos + 1) and 15

var state = TTest()
var accm: int64 = 0
for i in 1..100000000:
  accm = (next state)
  # discard (next state) works without issues