Gotchas related to prefix +/- operators

[tera]

Let's have a place to discuss issues related to prefix plus and prefix minus operators gotchas (unexpected behaviour) cause by prefix minus being parsed as part of the integer and float literal processing and due to other special treatments (like concrete overloads being preferred over generic overloads, etc). If you add an entry please specify whether it is compile time or run time (the latter is obviously more dangerous).

Even if we decide that the the existing state of affairs is ok with us this thread will raise awareness of the specific pitfalls and fewer people will get caught by surprise.

Run time gotchas:

Being runtime they are quite scary.

@xwu

// Run time
let x: Float = -0   // +0.0
let y: Float = -(0) // -0.0

---

@bbrk24

// Run time
var x: Double = 0
var y: Double = -0
let identical = (memcmp(&x, &y, MemoryLayout<Double>.size) == 0)

---

@tera

// Run time
let a: Double = -0
print(a.sign) // plus 🤔
let b: Double = -0.0
print(b.sign) // minus
let c: Double = -(0)
print(c.sign) // minus
let d: Double = -(0.0)
print(d.sign) // minus

---

// Run time
let x: Int8 = -(-128)
// 🤔 somewhat unexpected that this compiles (crashes at runtime)

---

@tera

// Run time
struct OnesComplement: ExpressibleByIntegerLiteral {
    init(integerLiteral value: Int) {
        self.value = value
    }
    init(value: Int) {
        self.value = value
    }
    var value: Int
    
    static prefix func - (a: Self) -> Self {
        return Self(value: ~a.value)
    }
}
let one: OnesComplement = 1
print("1 ==", one.value)        // 1 ==  1
let a: OnesComplement = -one
print("-2 ==", a.value)         // -2 == -2
let b: OnesComplement = -(1 as OnesComplement)
print("-2 ==", b.value)         // -2 == -2
let c: OnesComplement = -1
print("-2 ==", c.value)         // -2 == -1  🤔?!

---

Compile time gotchas

Being compile time they are not so dangerous.

Would be a gotcha if "-" was parsed as an operator:

@taylorswift

parsing -0 as (-)(0 as some ExpressibleByIntegerLiteral) breaks integer literal initializers. in particular, it will no longer be possible to express:

let min:Int8 = -128

---

@tera

// Compile time
enum SpecialNumbers: Double {
    case negativeZero = -0.0
    case zero = 0.0
    case positiveZero = +0.0 // 🛑 Enum case must declare a raw value when the preceding raw value is not an integer
}

---

Other similar gotchas

Unrelated to prefix + / - operators specifically, but potentially to all prefix operators:

@1-877-547-7272

// Run time (not +/- specific)
extension Numeric {
    func incremented() -> Self {
        return self + 1
    }
}
-13.incremented()   // -14
(-13).incremented() // -12
-(13.incremented()) // -14

---

// Run time (not +/- specific, same issue)
let x = -42
    .addingReportingOverflow(42).partialValue
    // if you are unfamiliar this is same as + (almost)
print(x) // -84 🤔

---

// Compile time (not +/- specific, same issue)
let x: Int8 = -128.self // 🛑 Integer literal '128' overflows when stored into 'Int8'