.init<Subject: CustomStringConvertible>(xxx instance: Subject) vs. .init(xxx instance: CustomStringConvertible)

[xwu (Xiaodi Wu)]

@young

// these both call 2
// case 1
_ = String.init(xxx: Foo())      // why the compiler not choose 1 here?
// case 2
_ = String.init(xxx: Foo() as CustomStringConvertible)

That's a very interesting question, and I'm actually not clear as to the answer. What's more interesting, this behavior isn't the same for a custom protocol:

protocol P { }
struct S: P { }
extension String {
    init<T: P>(xxx t: T) { fatalError("C") }
    init(xxx p: P) { fatalError("D") }
}
String(xxx: S()) // error: Ambiguous use of 'init(xxx:)'

I wonder if the behavior observed is unintentional. I actually don't recall this particular question coming up before, at least in the context of initializers. @dabrahams @jrose @Douglas_Gregor?