Not sure if this is a bug or not, but if you have a protocol with a mutating method:
protocol Rearrangeable: Collection {
mutating func move(from current: Index, to new: Index)
}
And define another AnyObject protocol that conforms to it:
protocol ThisIsNotAValueType: AnyObject, Rearrangeable {}
If you ever have a generic that conforms to the refined protocol, it won't permit calling the mutating method:
func here<T>(object: T) where T: ThisIsNotAValueType {
// code ...
object.move(from: current, to: new)
// Get error: "Cannot use mutating member on immutable value: 'object' is a 'let' constant"
}
This can be circumvented by redefining the mutating method requirement in the refined protocol:
protocol ThisIsNotAValueType: AnyObject, Rearrangeable {
/// - Note: Added as `Rearrangeable.move(from:to:)` is marked as `mutating`
/// and so doesn't work with immutable references; even though we're an
/// `AnyObject`.
func move(from current: Index, to new: Index)
}