An example of Extension based on Kingfisher in Swift.

ExtensionCompatibleSample.swift

public final class Extension<Base> {
    public let base: Base
    public init(_ base: Base) {
        self.base = base
    }
}

public struct StaticExtension<Base> { }

public protocol ExtensionCompatible {
    associatedtype Base
    var ex: Extension<Base> { get }
    static var ex: StaticExtension<Base> { get }
}

public extension ExtensionCompatible {
    public var ex: Extension<Self> {
        return Extension(self)
    }
    public static var ex: StaticExtension<Self> {
        return StaticExtension()
    }
}

/// Int Extension

extension Int: ExtensionCompatible { }

public extension Extension where Base == Int {
    public var cube: Int {
        return base * base * base
    }
}

public extension StaticExtension where Base == Int {
    public var almostMax: Int {
        return Int.max - 1
    }
}

4.ex.cube           // 64
Int.ex.almostMax    // 9223372036854775806


/// String Extension

extension String: ExtensionCompatible { }

public extension Extension where Base == String {
    public var reversed: String {
        return String(base.characters.reversed())
    }
}

public extension StaticExtension where Base == String {
    public func greet(to name: String) -> String {
        return "Hi, \(name)."
    }
}

"hello".ex.reversed             // olleh
String.ex.greet(to: "Michael")  // Hi, Michael.