Bug? Known? Protocol extension with settable computed property

[DevAndArtist (Adrian Meister)]

I just asked the same question a day ago, it‘s a little mind bending but it makes sense.

Why can we not mutate mutable members on a reference type?

I stumbled against this wall a few times, previously I simply ignored it, but now it seems like a pointless language limitation. Every object type (right now we only have classes) in Swift is implicitly a subtype of AnyObject, which in fact is like a type alias to the class keyword. That makes any instance that is known to be of type AnyObject a reference type. However when used in a protocol extension the compiler does not allow one mutate mutable members even if Self is guaranteed to be a Any…