struct S<A> {
    init(a: A) {
        self.a = a
    }

    private let a: A

    static func m(_ x: [S<A>]) -> S<A> {
        return x.first!
    }
}

extension S {
    func map<B>(_ f: (A) -> B) -> S<B> {
        // Is this another bug? This doesn't compile without the explicit <B>
        return S<B>(a: f(self.a))
    }
}

func f() -> S<Bool> {
    let x1 = S<Void>(a: ())
    let x2 = S<Void>(a: ())

    return S.m([
        x1.map { true },
        x2.map { false }
    ])
}