Why this struct shadows the type String?

Question

I am trying to understand property wrappers.

I have another question of mine on SO, where I was trying to create a property wrapper like this:

extension String {

  func findReplace(_ target: String, withString: String) -> String
  {
    return self.replacingOccurrences(of: target,
                                     with: withString,
                                     options: NSString.CompareOptions.literal,
                                     range: nil)
  }
}


  @propertyWrapper
  struct AdjustTextWithAppName<String> {
    private var value: String?


    init(wrappedValue: String?) {
      self.value = wrappedValue
    }

    var wrappedValue: String? {
      get { value }
      set {
        if let localizedAppName = Bundle.main.localizedInfoDictionary?["CFBundleName"] as? String {
          let replaced = value.findReplace("$$$", withString: localizedAppName)

        }
        value = nil
      }
    }

  }

That was not working because the line value.findReplace was showing an error

Value of type String? has no name findReplace

As soon as someone suggested me to change the struct line to

struct AdjustTextWithAppName {

the whole thing started working.

Why? I cannot understand why <String> term on the struct was shadowing the extension to the String type I have created.

Why is that?

Answer 1

Replace <String> with the common generic type <T> and you'll see the issue immediately

 @propertyWrapper
  struct AdjustTextWithAppName<T> {
    private var value: T?


    init(wrappedValue: T?) {
      self.value = wrappedValue
    }

    var wrappedValue: T? {
      get { value }
      set {
        if let localizedAppName = Bundle.main.localizedInfoDictionary?["CFBundleName"] as? String {
            let replaced = value.findReplace("$$$", withString: localizedAppName) // Value of type 'T' has no member 'findReplace'

        }
        value = nil
      }
    }
  }

Now the error is more understandable

Value of type 'T' has no member 'findReplace'

Answer 2

I cannot understand why <String> term on the struct was shadowing the extension to the String type I have created.

Why wouldn't it? You explicitly requested for there to be a generic type parameter to AdjustTextWithAppName called String . The compiler gave you precisely what you asked for.