Protocol vs generic type extensions - overload resolution

protocolextension.swift

// works for any index type
extension CollectionType {
    var mid: Index {
        return advance(startIndex,
            distance(startIndex,endIndex) / 2
        )
    }
}

// and specialize for random-access index types
extension CollectionType where Index: RandomAccessIndexType {
    var mid: Index {
        return startIndex.advancedBy(
            startIndex.distanceTo(endIndex) / 2
        )
    }
}

// string indices are bidirectional
let s = "hello"
let stringRange = s.startIndex..<s.endIndex

// so Swift calls the more general version
stringRange.mid

// but Ints is random-access
let intRange = 0..<10
// so Swift calls the more specialized version
intRange.mid

typeextension.swift

/// Now the same thing, but with an extension on Range instead of CollectionType

// works for any index type
extension Range {
    var mid: T {
        return advance(startIndex,
            distance(startIndex,endIndex) / 2
        )
    }
}

// and specialize for random-access index types
extension Range where T: RandomAccessIndexType {
    var mid: T {
        return startIndex.advancedBy(
            startIndex.distanceTo(endIndex) / 2
        )
    }
}

let s = "hello"
let stringRange = s.startIndex..<s.endIndex

// singe String.Index is bidirectional, no ambiguity
stringRange.mid

// but Ints being random-access
let intRange = 0..<10
// Swift complains error: ambiguous use of 'mid'
intRange.mid