Open Radar

 
Summary:
It looks like protocol extensions can't declare a conformance to a protocol. The following code seems to compile, but Swift Playground won't display any value, showing that it can run:

    protocol P { }
    extension CollectionType: P { }
    let p = [1] as! P

That would be nice, because it would allow protocol adoption to recursively cascade from simple types to collections and dictionaries of those types:

    protocol P { }
    extension CollectionType : P where Generator.Element : P { }
    extension DictionaryLiteralConvertible : P where Key : String, Value : P { }
    extension Int: P { }

    let a: P = 1
    let b: P = [1, 2, 3]
    let c: P = [[1, 2], [3, 4]]
    let d: P = ["foo": [[1, 2], [3, 4]]]
    let e: P = [["foo": 1], ["bar": 2]]

Steps to Reproduce:
See Description

Expected Results:
No compilation error, and the recursive adoption of a protocol.

Actual Results:
No recursive protocol adoption

Version:
Xcode Version 7.0 beta (7A120f)

Notes:
Please :-)