Swift for…in generic inference prob

GenericInfProb.swift

// original version with problem
struct A<T> {
    func f<S: SequenceType where S.Generator.Element == T>(s: S) {
        // error: cannot convert expression's type S to type S
        for v in s {
            print(v)
        }
    }
}

// manually convert for...in to generator and while loop
struct B<T> {
    func f<S: SequenceType where S.Generator.Element == T>(s: S) {
        var g = s.generate()
        // error: cannot convert the expression's type '()' to type 'Self.Element??'
        while let v = g.next() {
            print(v)
        }
    }
}

// explicit type in while loop solves the problem
struct C<T> {
    func f<S: SequenceType where S.Generator.Element == T>(s: S) {
        var g = s.generate()
        // annotating v to be of type T fixes this
        while let v: T = g.next() {
            print(v)
        }
    }
}

// prints 123
let c = C<Int>()
c.f([1,2,3])

// but same solution doesn't work for the for...in version
struct D<T> {
    func f<S: SequenceType where S.Generator.Element == T>(s: S) {
        // error: Type of expression is ambiguous without more context
        for v: T in s {
            print(v)
        }
    }
}