struct Foo<T> {
    let v: T
    init(_ v: T) { self.v = v }
}
typealias Bar = Foo<(Int, Int)>
struct Baz {
    let v: (Int, Int)
    init(_ v: (Int, Int)) { self.v = v }
}
// Study the above and this:
// (I'm not counting argument list parens here):
let f = Foo(1, 2)   // Accepts ZERO OR MORE parens around 1, 2.
let b = Bar(1, 2)   // Accepts ONLY ZERO    parens around 1, 2.
let c = Baz((1, 2)) // Accepts ONE OR MORE  parens around 1, 2.
// You can verify that the rules above are correct by adding and
// removing parens.
// Evidently, Foo<(Int, Int)>, Bar and Baz are different (in that
// their initializers can not be used in the same way).
// But why/how?