CODEWITHSUNDEEP
swiftgenericsprotocolsrestriction
Ed Connell
Ed Connell

Reputation: 53

swift inconsistent generic protocol restriction

I seem to be running into what appears to be a compiler inconsistency when passing arguments to a generic function that has a protocol restriction. I can pass a concrete argument, but not an argument as a protocol type

protocol Selectable {
    func select()
}

protocol Log : Selectable {
    func write()
}

class DefaultLog : Log {
    func select() {
        print("selecting")
    }
    func write() {
        print("writing")
    }
}

let concrete = DefaultLog()
let proto: Log = DefaultLog()

func myfunc<T: Selectable>(arg: T) {
    arg.select()
}

myfunc(concrete)   // <-- This works
myfunc(proto)      // <-- This causes a compiler error
proto.write()      // <-- This works fine

The compiler reports:

error: cannot invoke 'myfunc' with an argument list of type '(Log)'
myfunc(proto)
^
note: expected an argument list of type '(T)'
myfunc(proto)
^

If I restrict the function to the Selectable or Log protocol it still fails.

Is this a compiler bug? Any thoughts?

Upvotes: 3

Views: 61

Answers (1)

Lou Franco
Lou Franco

Reputation: 89232

If you are using a protocol, it doesn't need to be generic:

func myfunc(arg: Selectable) {
    arg.select()
}

I believe that T needs to be a concrete type for generics.

Upvotes: 1

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