protocol ExpressibleByNilLiteral A type that can be initialized using the nil literal, nil. Conforming Types Optional nil has a specific meaning in Swift---the absence of a value. Only the Optional type conforms to ExpressibleByNilLiteral. ExpressibleByNilLiteral conformance for types that use nil for other purposes is discouraged. Initializers init init(nilLiteral:) Required Creates an instance initialized with nil. Declaration init(nilLiteral: ())