Tagged String - "Ambiguous use of "init(_:)'"

[philipbel]

Consider the following:

enum _StringTag { }
typealias MyString = Tagged<_StringTag, String>

let aString = "bar"
let myString = MyString(aString)

Compiler complains that the MyString.init() call is ambiguous. I would expect this to just work, since I am passing a String where RawValue == String.

[...]:18:16: error: ambiguous use of 'init(_:)'
let myString = MyString(aString)
               ^
[...]/Tagged.swift:9:10: note: found this candidate
  public init(_ rawValue: RawValue) {
         ^
[...]/Tagged.swift:199:10: note: found this candidate
  public init?(_ description: String) {

And indeed, from Tagged.swift:

[...]
  public init(_ rawValue: RawValue) {
    self.rawValue = rawValue
  }
[...]
extension Tagged: LosslessStringConvertible where RawValue: LosslessStringConvertible {
  public init?(_ description: String) {
    guard let rawValue = RawValue(description) else { return nil }
    self.init(rawValue: rawValue)
  }
}

[mbrandonw]

Hi @philipbel, this is just an unfortunate consequence of trying to support a succinct initialization syntax (e.g. Tagged(value)), and some conveniences, like LosslessStringConvertible.

But at the end of the day you can always disambiguate by just using the more verbose Tagged.init(rawValue:):

swift-tagged/Sources/Tagged/Tagged.swift

Lines 5 to 7 in 8ce7c8c

	public init(rawValue: RawValue) {
	self.rawValue = rawValue
	}

There is also chance using @_disfavoredOverload on the LosslessStringConvertible initializer would have also helped. If you want to PR that we will consider adding it to the library.

But since this isn't really an issue with the library I am going to convert it to a discussion. please feel free to continue the conversation over there.