Open Radar

 
Summary:
Having a protocol P and a protocol T, it would be useful to express that any type P also conforms to T by providing an extension.

Steps to Reproduce:
- Compile the following code:

protocol T {}
protocol S {}

extension T: S {}

Expected Results:
- It compiles.

Actual Results:
- error: extension of protocol 'T' cannot have an inheritance clause

Notes:
- This would be extra useful in addition to rdar://23616223.
- A more useful example could be:

protocol SwiftProtocol {
	var swiftValue: SwiftOnlyType { get }
}

public protocol ObjectiveCProtocol {
	var objectiveCValue: ObjectiveCType { get }
}

extension ObjectiveCProtocol: SwiftProtocol {
	var swiftValue: SwiftOnlyType { 
		return self.objectiveCValue.bridgedToSwift()
	}
}