How to use a protocol with optional class methods in an extension with generic in Swift?

Question

I am trying using extension for an existing class with class method like:

@objc public protocol MyProtocol {
    optional class func foo() -> Int
}

And I am using this protocol in an extension with generic like:

extension MyClass {

    public func bar<T: MyProtocol>() {
        ...
        let x: Int = T.self.foo!() // if I do not use "self" or "!" here, I will have a compiler error
        ...
    }

This should work but when I build it, Xcode says "Command /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swiftc failed with exit code 1". If I do not use "optional" in the protocol, I do not need to unwrap the foo() in the extension and everything works well even if I remove the "self". Can anyone tell me why and how to make the optional work properly? Thanks in advance.

Answer 1

It looks like you've found a (fairly obscure) bug in the Swift compiler that's causing it to crash. Here's a reproduction that's all you need in a single file to crash swiftc:

import Foundation
@objc protocol P { optional class func f() }
func f<T: P>(t: T) { T.self.f?() }

(you don't need to call f for it to crash)

You should probably file a radar since the compiler crashing is never expected behavior no matter what your code.

If you tried to do this without the optional, it'll work (and you can even ditch the self). My guess is the implementation of generics doesn't currently account for the possibility of optional class-level functions.

You can do it without generics like this:

func f(p: MyProtocol) {
    (p as AnyObject).dynamicType.foo?()
}

(there may even be a better way but I can't spot it).

You need the AnyObject cast because if you try to call .dynamicType.foo?() on p directly you get "accessing members of protocol type value 'MyProtocol.Type' is unimplemented". I wouldn't be surprised if the crash of the generic version is related to this.

I'd also say that its worth asking yourself whether you really need a protocol with optional methods (especially class-level ones) and whether there's a way to do what you want entirely statically using generics (as you're already semi-doing).