protocol P {
  associatedtype Header
  var header: Header { get set }
}

extension P {
  mutating func f() {
    withUnsafePointer(to: &self.header) { print($0) }
  }
}

class X : P {
  var header: Int = 0
  init() {
    withUnsafePointer(to: &self.header) { print($0) }

    // This works:
    var me = self
    me.f()
    
    // But this doesn't:
    self.f() // error: cannot use mutating member on immutable value: 'self' is immutable
  }
}

X()