[swift-evolution] [swift-users] Very strange automatic behavior between StringLiteralConvertible and pattern matching

[David Hart]

To bring a little bit more context: I copied this Regex library in my project which had StringLiteralConvertible and implemented the pattern matching operator and all of a sudden, ALL init(rawValue: String) calls of completely unrelated enums started returning unexpected values. If I did not have unit tests, I probably would not have found out about it for a while.

I understand the mechanism which calls StringLiteralConvertible's init(stringLiteral) under the hood:

let a: SomeStringLiteralConvertibleType = "hello"

I also understand the magic in the pattern matching operator. But I don't understand why when associating them together:

func ~=(lhs: SomeStringLiteralConvertibleType, rhs: String) -> Bool {
    return false
}

Then creates his behaviour in all Enums with String raw values:

enum MyEnum: String {
    case Super = "super"
}

let a = MyEnum(rawValue: "super") // nil

I can't figure out if this is just a confusing behaviour of Swift, in which case I want to write a proposal to make it less confusing, or if it is a big with Swift, in which case I should open a bug report.

David

> On 05 Jan 2016, at 18:26, David Hart via swift-users <swift-users at swift.org> wrote:
> 
> How is it that Swift allows code like this:
> 
> struct Sneaky: StringLiteralConvertible {
> 	init(stringLiteral value: String) {}
> 	init(extendedGraphemeClusterLiteral value: String) {}
> 	init(unicodeScalarLiteral value: String) {}
> }
> 
> func ~=(sneaky: Sneaky, string: String) -> Bool {
> 	return false
> }
> 
> enum NormalEnum: String {
> 	case Super = "super"
> 	case Mario = "mario"
> }
> 
> let value = NormalEnum(rawValue: "super”) // return nil!!!!
> 
> It hit completely by surprise today because of of a Regex library:
> 
> struct Regex: StringLiteralConvertible {
> 	init(stringLiteral value: String) {}
> 	init(extendedGraphemeClusterLiteral value: String) {}
> 	init(unicodeScalarLiteral value: String) {}
> 
> 	//...
> }
> 
> func ~=(regex: Regex, string: String) -> Bool {
> 	return regex.matches(string)
> }
> 
> If I was not already a Swift enthusiast, this behaviour would have left me completely dumbfounded.
> What can we do about it?
> 
> David.
> 
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> swift-users at swift.org
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