[swift-users] A confusing protocol extension behaviour/bug

[Howard Lovatt]

Unfortunately the error messages you get with protocols are limited :). The compiler is doing the right thing, no bug. The problem is that it is really easy to mak

-- Howard. 

> On 1 Jan 2018, at 3:40 pm, Toni Suter via swift-users <swift-users at swift.org> wrote:
> 
> Hi Marc,
> 
> There are several subtleties here, but I think the compiler is actually doing the right thing.
> 
> The second class defines a static property that looks like it is 'overriding' the static property from
> the protocol extension, but since the types don't match (String vs. String?), it sort of 'overloads'
> the property (similar to function overloading). Nevertheless, the class still fulfills the requirements
> of the Trackable protocol, by inheriting the static property from the protocol extension.
> 
> When you access analyticsID like a regular static property, the Swift compiler will choose the String property,
> because it shadows the String? property:
> 
> let x = Something2.analyticsID
> print(x)					// Wrong but compilers, returns wrong value
> print(type(of: x))				// String
> 
> However, when the context of the expression Something2.analyticsID expects a String?, the Swift compiler will
> choose the String? property:
> 
> let a: String? = Something2.analyticsID		// explicit type annotation demands a String?
> print(a)					// nil
> print(type(of: a))				// Optional<String>
> 
> let b = Something2.analyticsID as String?	// type cast demands a String?
> print(b)					// nil
> print(type(of: b))				// Optional<String>
> 
> A similar thing happens, when you write Something2.analyticsID ?? "nil". The nil coalescing operator ?? demands that the first parameter
> is an optional. Therefore, the Swift compiler will choose the String? property instead of the String property.
> 
> I hope this helps!
> 
> Best regards,
> Toni
> 
>> Am 01.01.2018 um 18:29 schrieb Marc Palmer via swift-users <swift-users at swift.org>:
>> 
>> Hi,
>> 
>> I hope everybody had a great New Year celebration.
>> 
>> I was tracking down a weird bug in my Swift code today. A property defined in a class in order to conform to a protocol was not being seen. A protocol extension provided a default value of `nil` for this property, so I knew where it was coming from. Turned out, in my class I had defined the property with the correct name but incorrect type - I declared it as `String` instead of `String?`. 
>> 
>> I isolated this behaviour in a playground, shown below, and it is pretty weird behaviour.
>> 
>> The output is:
>> 
>> Something1 has id: nil
>> Something2 has id: nil
>> Something3 has id: Correct
>> -- Direct access--
>> Something1 - nil
>> Something2 - nil
>> Something2 with String(describing:) - Wrong but compiles, returns wrong value
>> Something3 - Correct
>> 
>> The playground code:
>> 
>> ======================
>> 
>> protocol Trackable {
>>    static var analyticsID: String? { get }
>> }
>> 
>> extension Trackable {
>>    static var analyticsID: String? { return nil }
>> }
>> 
>> class Something1: Trackable {
>> }
>> 
>> class Something2: Trackable {
>>    static var analyticsID: String = "Wrong but compiles, returns wrong value"
>> }
>> 
>> class Something3: Trackable {
>>    static var analyticsID: String? = "Correct"
>> }
>> 
>> func getID<T: Trackable>(_ trackable: T.Type) {
>>    if let id = trackable.analyticsID {
>>        print("\(trackable) has id: \(id)")
>>    } else {
>>        print("\(trackable) has id: nil")
>>    }
>> }
>> 
>> getID(Something1.self)
>> getID(Something2.self)
>> getID(Something3.self)
>> 
>> print("-- Direct access--")
>> print("Something1 - \(Something1.self.analyticsID ?? "nil")")
>> print("Something2 A - \(Something2.self.analyticsID ?? "nil")")
>> print("Something2 with String(describing:) - \(String(describing: Something2.self.analyticsID))")
>> print("Something3 - \(Something3.self.analyticsID ?? "nil")”)
>> ======================
>> 
>> Thanks in advance for any information about my misinterpretations or recommendations of what parts are actually undesirable so that I can raise the JIRAs.
>> 
>> Cheers
>> 
>> —
>> Marc Palmer
>> Montana Floss Co. Ltd.
>> 
>> Soundproof – Music Player for Practice 
>> http://getsoundproof.com
>> 
>> 
>> 
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> 
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