JuniorTheoryCommon

What happens when this program is compiled and run?

SWIFT

class Box {
    var n = 0
}

let b = Box()
b.n = 5
print(b.n)

Expecting a compile error, as with a struct in a let: for a class, let fixes only the reference, so the object's var properties still change.
Expecting 0, as if b.n = 5 changed a copy: b leads to the object itself, so the change stays in it.