Should frames and coordinates have a distance atttribute?

[mhvk]

Right now, they only have it if their representation is spherical, but it would seem generally useful.

A side benefit would be that this new property could guarantee a Distance instance, thus removing the need to explicitly set anything with units of length in SphericalRepresentation to a distance.

[astrojuanlu]

If I understand correctly, you suggest moving the validation of Distance from SphericalRepresentation to BaseCoordinateFrame and SkyCoord somehow?

[mhvk]

Even now the only real validation it does is checking values are not negative, so I'm not sure how much we lose there.

My proposal was mostly for the output side: right now, you can easily do frame.distance.kpc and having a distance property would mean this remains true even if the underlying SphericalRepresentation has a Quantity for the distance.