message: recorded vcall: complain about uninitialized inputs
diff: @@ -32,8 +32,11 @@ void read_indices(uint32_t *out, uint32_t &count, const T &value) {
  } else if constexpr (is_diff_array_v<T>) {
  read_indices(out, count, value.detach_());
  } else if constexpr (is_jit_array_v<T>) {
  + uint32_t i = value.index();
  + if (i == 0)
  + jitc_fail("enoki::detail::read_indices(): uninitialized variable!");
  if (out)
  - out[count] = value.index();
  + out[count] = i;
  count += 1;
  } else if constexpr (is_enoki_struct_v<T>) {
  struct_support_t<T>::apply_1(
  