typealias Closure<A, B> = ((a: A, b: B)) -> String

func invoke<A, B>(a: A, b: B, _ closure: Closure<A,B>) {
  print(closure((a, b)))
}

// error: value of tuple type '(_, _)' has no member 'b'
invoke(a: 1, b: "B") { $0.b }

// this works
invoke(a: 1, b: "B") { $0.1 }

// this works
invoke(a: 1, b: "B") { (c: (a: Int, b: String)) in
  return c.b
}

// this doesn't work: error: value of tuple type '(_, _)' has no member 'b'
invoke(a: 1, b: "B") { c in
  return c.b
}