SkyCoord using frames not in the transform graph #5882
Description
As noted by @eteq, in principle it should be possible to use SkyCoord with frames not in the transform graph, and still have access to their frame attributes, i.e., the following tests should pass:
def test_transformless_frame():
"""
Checks the behavior of SkyCoord's with a frame that isn't in the transform
graph
"""
class TestFrame(BaseCoordinateFrame):
fattr = FrameAttribute()
default_representation = SphericalRepresentation
tf = TestFrame(1*u.deg, 2*u.deg, fattr='sillywalks')
sc = SkyCoord(tf)
assert tf.fattr == sc.fattr
sc2 = SkyCoord(3*u.deg, 4*u.deg, fattr='sillywalks2', frame=TestFrame)
assert sc2.frame.fattr == sc2.fattr