InterpolationDefault.swift

extension String.StringInterpolation {
    mutating func appendInterpolation<T>(_ value: T?, default: @autoclosure () -> String) {
        self.appendLiteral(value.map(String.init(describing:)) ?? `default`())
    }
}

let num = Int("21")
print("The number is: \(num, default: "<invalid>")")
// The number is: 21

let num2 = Int("asd")
print("The number is: \(num2, default: "<invalid>")")
// The number is: <invalid>