WCS handles negative slice index incorrectly #8771

WCS objects can be sliced, with some restrictions (e.g. no negative steps). Currently, wcs.slice returns nonsensical results if the start of the slice is negative.

I don't know that WCS should be able to handle negative start since those require knowing the number of dimensions along each axis, and discussion of that topic has been vigorous in the past (see #4669 and #5461). My particular use case is with CCDData (well, really, NDSlicingMixin) where there will always image dimensions and a negative index can be converted to positive.

The example below demonstrates the issue.

from astropy.wcs import WCS

wcs = WCS(naxis=2)
wcs.wcs.crpix = [5, 5]
wcs.wcs.crval = [0.0, 1.0]
wcs.wcs.cdelt = [1.0, 1.0]

wcs_good_slice = wcs[4:, 4:]
# The slice below would, for an array, give the same thing
# as the slice above.
wcs_bad_slice = wcs[-6:, -6:]

print('crval: ', wcs.wcs.crval)
print('coordinate value at pixel [0, 0] (should be crval): ', wcs_good_slice.all_pix2world([[0, 0]], 0))
print('coordinate value at pixel [0, 0] (should be crval): ', wcs_bad_slice.all_pix2world([[0, 0]], 0))