This was created to show an immutability error with swift properties declared in a protocol and access in a protocol extension

Protocol-Immutability.swift

protocol Generator
{
    typealias ObjectType: Generatable
    
    var objects: [ObjectType] { get set }
    mutating func objectForDeltaTime(seconds: CFTimeInterval) -> ObjectType?
}

extension Generator
{
    mutating func objectForDeltaTime(seconds: CFTimeInterval) -> ObjectType?
    {
        // This does not work
        //  error: immutable value of type 'Self.ObjectType' only has mutating members named 'removeAtIndex' 
        let newObject = objects.removeAtIndex(0)
        
        // This works
        var objs = objects
        let newObject = objs.removeAtIndex(0)
        objects = objs
        
        return newObject
    }
}

Note to anyone looking at this, the line marked "This does not work" does in fact work. The compiler was playing tricks on me and I kept his here for historic reasons. Since objects is a var, you can mutate it directly.