ExpressibleByNilLiteral
A type that can be initialized using the nil literal, nil.
Declaration
protocol ExpressibleByNilLiteral : ~Copyable, ~EscapableOverview
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.