Interface for determining and scoping optional Associated Types

Optional.swift

/// Interface representing an optionally nil value type.
protocol OptionalType: ExpressibleByNilLiteral {
    /// The boxed optional type.
    associatedtype Wrapped
}

extension Optional: OptionalType {}

// Example

// In this case, we have an extension that returns an optional type.
// If the associated type is defined as optional, you will return a double optional.
// The following example shows how you can use the `OptionalType` interface to scope
// an extension containing an associated type defined as optional. E.g. `InputType??`

protocol Validation {
  associatedType InputType
}

// Returns `InputType?` 
extension Validation {
  func validate() -> InputType? {}
}

// Returns `InputType?` 
extension Validation: InputType {
  func validate() -> InputType {}
}