Type promotion

Promote.swift

// fill in the blanks with mismatched lhs and rhs types
func +(lhs: Int, rhs: Double) -> Double {
    return Double(lhs) + rhs
}

func +(lhs: Double, rhs: Int) -> Double {
    return lhs + Double(rhs)
}

func +(lhs: Int, rhs: Float) -> Float {
    return Float(lhs) + rhs
}

func +(lhs: Float, rhs: Int) -> Float {
    return lhs + Float(rhs)
}

let r = Int(1) + Double(2)