Should the unit type U in Quantity[N, U] be covariant?

[erikerlandson]

trait Fruit
trait Apple extends Fruit
trait Pear extends Fruit

val q1 = 1.withUnit[Apple] + 1.withUnit[Pear]  // is q1 Quantity[Int, Fruit] ?

def f1(f: Quantity[Int, Fruit]) = { ... }

f1(1.withUnit[Apple])
f1(1.withUnit[Pear])

would likely depend on #22

[erikerlandson]

Although this is interesting, my attempts so far to make it work have failed, and it would be a major increase in implementation complexity, and also increase difficulty to reason about coulom's behavior, so I'm closing this.