7> func safePrint<T: CustomStringConvertible>(_ value: T) { print(value) } 8> safePrint(a) error: repl.swift:8:11: error: value of optional type 'Int?' not unwrapped; did you mean to use '!' or '?'? safePrint(a) ^ (edited) 7> func safePrint<T: CustomStringConvertible>(_ value: T) { print(value) } 8> safePrint(a) error: repl.swift:8:11: error: value of optional type 'Int?' not unwrapped; did you mean to use '!' or '?'? safePrint(a) ^ (edited)Optional は CustomStringConvertible ではないけど CustomDebugStringConvertible ではある（↑）。 (edited)