do {
    struct S {
        var x: Int
        
        static func +(lhs: S, rhs: S) -> S {
            return S(x: lhs.x + rhs.x)
        }
    }
    
    let s1 = S(x: 1)
    let s2 = S(x: 2)
    print(s1 + s2)    // error: binary operator '+' cannot
                      // be applied to two 'S' operands
}

do {
    struct S: Equatable {
        static func ==(lhs: S, rhs: S) -> Bool {
            return false
        }
    }
    
    print(S() == S()) // true (but should be false)
}