propensive
Restrict usage of offset quantities
There's currently nothing to stop you adding two Celsius quantities together, even though it probably doesn't make sense. The reason is that a figure quoted "in Celsius", without more context, could mean two different things: either an absolute temperature, or as a difference between two absolute temperatures. The latter should rightly be quoted in Kelvin, and never as an offset quantity. (The same applies to Fahrenheit.)
For clarity, such numbers should not be Quantitys at all. They should have a different type, which we can call Measurement. Each Measurement should correspond to a set of units (as with Quantity) but needs an artificial origin. In affine-space terminology they are points whereas Quantitys are vectors.