nditer usage with index and buffered flags

[rschiewer]

When using a nditer with any kind of index flag like 'c_index', 'f_index' or 'multi_index', the index will always be zero if the 'buffered' flag is used as well. If this is intended behavior, it does not seem to be documented though.

Reproducing code example:

a = np.arange(4*4*3).reshape(4,4,3)
it = np.nditer(a, flags=['multi_index', 'buffered'])
while not it.finished:
    print(it.multi_index)
    it.iternext()

Numpy/Python version information:

numpy version: 1.17.0
python version: 3.7.4 (default, Jul 9 2019, 18:13:23) [Clang 10.0.1 (clang-1001.0.46.4)]

[seberg]

It seems like this is unsupported on the C-side, and should probably checked and raise an error when given (or fixed, but I expect fixing it could be a bit more involved, because the buffer machinery probably uses something like external_loop flag always).