[mpu]

Isn't it possible to talk about variance as soon as you have a type constructor (function from types to types)? In this case [].

Edit: I don't know go, but if slices give write access, I think the automatic conversion the author wants would be unsound.

[pcwalton]

Yes, a coercion would be unsound (at least, unless the coercion copied the backing store of the slice, which would be very unintuitive). It shouldn't coerce. I'm describing generic type parameter bounds, which are not the same thing.

(For what it's worth, I'm not sure I would bother solving this problem if I were suddenly put in charge of Go's design, given Go's extreme aversion to type system features. I'm just describing what the solution to this problem typically is.)