Adding/subtracting ABmag Quantities loses the "type" of magnitude

The following code raises a UnitConversionError, because it appears the math operation lost track of the “type” of magnitude. fluxMag and color are both ABmag, so I would expect their difference to also be an ABmag.

import numpy as np
import astropy.units as u

# color = np.random.random(5)*u.ABmag
color = 10*u.ABmag
flux = 10000
fluxMag = (flux*u.nJy).to(u.ABmag)
diff = fluxMag - color
print(color, fluxMag, diff)
print(diff.to(u.nJy))

prints the following, and then raises:

10.0 mag(AB) 21.4 mag(AB) 11.399999999999999 mag
...
astropy.units.core.UnitConversionError: '' (dimensionless) and 'nJy' (spectral flux density) are not convertible

If the - is changed to +, the exception is different:

10.0 mag(AB) 21.4 mag(AB) 31.4 mag(AB2)
...
astropy.units.core.UnitConversionError: 'AB2' and 'nJy' (spectral flux density) are not convertible

[parejkoj]

PR #8519 is an attempt at making the message better. I will try to clean it up to merge at the next coworking morning telecon.

[parejkoj]

Sorry about that!

python 3.6.6 numpy 1.14.5 astropy 3.0.3