SwiftCompilerOddity.swift

// Library code
class Model { }
protocol Fetchable {
    typealias FetchableType = Self
    static func fetchAll(sql: String) -> [FetchableType]
}

extension Fetchable where Self : Model, FetchableType == Self {
    static func fetchAll(sql: String) -> [FetchableType] {
        return []   // bare minimal, for demonstration purpose
    }
}

// Userland code
class Parent: Model, Fetchable { }
class Child: Parent { }

let parents = Parent.fetchAll("SELECT * FROM ...") // [Parent]

// error: cannot invoke 'fetchAll' with an argument list of type '(String)'
let children = Child.fetchAll("SELECT * FROM ...") // [Children]