Enum setter error reporting seems too simplistic?

bug.swift

enum E {
    final class P { var v = 0 }

    case a(P)

    var p: P {
        switch self {
        case .a(let p): return p
        }
    }

    var v: Int {
        get { return self.p.v }
        set { self.p.v = newValue }
    }

    func setV(_ newV: Int) {
        self.p.v = newV
    }
}

let a = E.a(E.P())
a.setV(1) // okay
a.v = 2 // error: cannot assign to property: 'a' is a 'let' constant