Take the two snippets below, the top one works fine but the bottom one results in Ambiguous use of 'subscript(_:)'

using index ✅

extension Array {
    subscript(i index: Int) -> (String, String)? {
        guard let value = self[index] as? Int else {
            return nil
        }
        
        switch (value >= 0, abs(value % 2)) {
        case (true, 0): return ("positive", "even")
        case(true, 1): return ("positive", "odd")
        case(false, 0): return ("negative", "even")
        case(false, 1): return ("negative", "odd")
        default: return nil
            
        }
    }
}

Without using index ❌

extension Array {
    subscript(i: Int) -> (String, String)? {
        guard let value = self[i] as? Int else {
            return nil
        }
        
        switch (value >= 0, abs(value % 2)) {
        case (true, 0): return ("positive", "even")
        case(true, 1): return ("positive", "odd")
        case(false, 0): return ("negative", "even")
        case(false, 1): return ("negative", "odd")
        default: return nil
            
        }
    }
}

CodePudding user response：

First, the name index is irrelevant to the problem; it could be any name.

The actual problem is that Array already has a subscript that takes an unlabeled Int.

Your first overload does not have the same input signature. Instead, it requires an argument label:

[1][i: 0] // ("positive", "odd")

You can still use an overload without a label…

extension Array where Element: BinaryInteger {
  subscript(