duplicates:

• SR-7327 Conditional conformance does not conform to inherited protocol

is duplicated by:

• SR-6914 Extensions don't add conformance to inherited protocols
• SR-7253 Conditional conformance to P fails if P inherits from two other protocols, but not for eg Array

relates to:

• SR-6840 Confusing error in conditional conformance
• SR-6650 Conditional conformance not identifying protocol extension implementation
• SR-6922 [SE-0143] [release mode] type ... does not conform to protocol ...

Issue Description:

The following code fails to compile with the 2017-11-24 snapshot.

protocol P {
  var count: Int { get }
}

protocol Q : P { }

// ---

struct S<T> { }

extension S : Q where T : Equatable {
  var count: Int { return 1 }
}

// ---

func f<T : P>(_ p: T) {
  print(p.count)
}

f(S<Int>())

Changing extension S : Q where... to extension S : P where... allows the code to compile, but of course S : Q should imply S : P.

Meanwhile, that workaround never permits S to conform to Q. That is, the following fails to compile:

struct S<T> { }

extension S : P where T : Equatable {
  var count: Int { return 1 }
}

extension S : Q where T : Equatable { }

(The practical upshot of this is that StrideTo cannot conditionally conform to RandomAccessCollection because that protocol exists in a hierarchy refining other protocols.)