ProtocolShenanigans.swift

protocol Able {
    init?(foo: String)
}

protocol Ible {
    init(bar: Int) throws
}

extension Able where Self: Ible {
    init?(foo: String) {
        // The bug can still be reproduced without the contents of this method
        // by just `return nil`.
        guard let value = Int(foo) else { return nil }
        guard let object = try? Self(bar: value) else {
            return nil
        }
        self = object
    }
}

struct Baz {
    let buzz: Int
}

extension Baz: Able { }

extension Baz: Ible {
    init(bar: Int) throws {
        self.buzz = bar
    }
}

let x = Baz(foo: "1") // Errors with: '(foo: String) -> Baz' is not convertible to '(foo: String) -> Baz?'
let y = Baz.init(foo: "1") // Works!

Playground execution failed: MyPlayground.playground:31:23: error: '(foo: String) -> Baz' is not convertible to '(foo: String) -> Baz?'
        guard let x = Baz(foo: foo) else { return nil }

Hm .. Baz implements Able and Ible. Seems like Baz(foo: "1") takes the init of Able which is init?
And I guess when you call Baz.init(foo: "1") it is explicit

You have a very strange construct there :)