Disabling overflow checking works for procs but not for blocks within a when isMainModule block?

error_checking_at_runtime.nim

{.push checks: off.}
proc unsafe_mult2(x: int32): int32 =
  result = x * 2
{.pop.}

{.push checks: off.}
proc unsafe_assert(x: int): void =
  assert x == (x + 1)
{.pop.}

when isMainModule:
  var x, y, z: int32
  x = 2147483647'i32  # 2**31 - 1

  # First use the proc versions
  try:
    let x_unsafe = unsafe_mult2(x)
    echo("No overflow when checks: off declared around the proc.")
  except EOverflow:
    echo("Overflow!")

  try:
    unsafe_assert(10)
    echo("Unsafe assert works when checks: off declared around the proc.")
  except EAssertionFailed:
    echo("Unsafe assert doesn't work.")

  # Now look at disabling exceptions w/in the isMainModule block directly
  try:
    y = x * 2
  except EOverflow:
    echo("Overflow!")

  # Still overflows
  {.push checks: off.}
  try:
    z = x * 2
    echo("No overflow!")
  except EOverflow:
    echo("Overflow!")
  {.pop.}

  # Still overflows
  {.push checks: off.}
  block within_a_block:
    try:
      z = x * 2
      echo("No overflow!")
    except EOverflow:
      echo("Overflow!")
  {.pop.}

  # Assertion fails
  try:
    assert 1 == 2
  except EAssertionFailed:
    echo("In what world? (Assertion checking on)")

  {.push checks: off.}
  try:
    assert 1 == 2
    echo("Assertion checking successfully turned off.")
  except EAssertionFailed:
    echo("In what world?")
  {.pop.}