4> a is CustomDebugStringConvertible $R2: Bool = true 5> a.description error: repl.swift:5:1: error: value of optional type 'Int?' not unwrapped; did you mean to use '!' or '?'? a.description ^ ? 5> func debugPrint<T: CustomDebugStringConvertible>(_ value: T) { print(value) } 6> debugPrint(a) Optional(42)