swift convenience init and generic class - swift

Question

I have a problem creating a convenience init method that then calls a designated init on a class with generic type parameters. Here is the swift 3.1 XCode Version 8.3.2 (8E2002) playground
protocol A {
var items: [String] { get set }
func doSomething()
}
struct Section : A {
var items: [String] = []
func doSomething() {
print("doSomething")
items.forEach { print($0) }
}
}
class DataSource<T: A> {
var sections: [T]
init(sections: [T]) {
self.sections = sections
}
func process() {
sections.forEach { $0.doSomething() }
}
convenience init() {
var section = Section()
section.items.append("Goodbye")
section.items.append("Swift")
self.init(sections: [section])
}
}
/*: Client */
var section = Section()
section.items.append("Hello")
section.items.append("Swift")
let ds = DataSource(sections: [section])
ds.process()
If no convenience init exists, then the code beneath the /*: Client */ section compiles and executes without issue. If I add in the convenience init I get the following compilation error:
cannot convert value of type '[Section]' to expected argument type '[_]'
self.init(sections: [section])
I wouldn't think that this would be an issue since in the convenience init I am creating a Section struct which implements the protocol A which satisfies the generic constraint on the DataSource class. The convenience init is performing the same operations as the client code, yet it is unable to convert a [Section] into a [A]. Is this an initialization sequencing issue?

Answer 1

Generic placeholders are satisfied at the usage of the given generic type – therefore inside your convenience init, you cannot assume that T is a Section. It's an arbitrary concrete type that conforms to A.
For example, it would be perfectly legal for the caller to define a
struct SomeOtherSection : A {...}
and then call your convenience initialiser with T being SomeOtherSection.
The solution in this case is simple, you can just add your convenience initialiser in an extension of DataSource, where T is constrained to being Section – therefore allowing you to call init(sections:) with a [Section]:
extension DataSource where T == Section {
convenience init() {
var section = Section()
section.items.append("Goodbye")
section.items.append("Swift")
self.init(sections: [section])
}
}
// ...
// compiler will infer that T == Section here.
let ds = DataSource()